R2: load-test harness + contour resource observability

New scrabble/loadtest module (the pre-release stress harness): seeds 1000 guest +
10000 durable accounts with pre-created sessions directly in Postgres (token hash
matches backend/internal/session), drives virtual players through the edge protocol
(real 2-4p games assembled via invitations, mid-ranked legal moves generated locally
by the embedded scrabble-solver — the edge carries no board, so the client replays
history), plus nudge/chat/check-word/draft/profile/stats and a gateway-hammer that
verifies the rate limiter. Prints a trip-report summary (per-op latency percentiles,
result codes, live-event tally). Go unit tests cover the pure pieces; the DAWG-backed
move test runs under BACKEND_DICT_DIR.

Contour: add cAdvisor + postgres_exporter + a 'Scrabble - Resources' Grafana
dashboard and the two Prometheus scrape jobs, for the R2/R7 stress-run resource
baseline.

CI: gate ./loadtest/... (path filter + vet/build/test). Docs: TESTING, ARCHITECTURE,
project CLAUDE repo layout.

loadtest/internal/edge/encode.go

@@ -0,0 +1,184 @@
+package edge
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+
+	fb "scrabble/pkg/fbs/scrabblefb"
+)
+
+// PlayTile is one tile to place, addressed by alphabet index (255 marks a blank's
+// carrier letter together with Blank=true), as the submit-play request carries it.
+type PlayTile struct {
+	Row, Col int
+	Letter   byte
+	Blank    bool
+}
+
+// gameAction builds a GameActionRequest payload (just a game id): pass, nudge,
+// history, draft.get.
+func gameAction(gameID string) []byte {
+	b := flatbuffers.NewBuilder(64)
+	gid := b.CreateString(gameID)
+	fb.GameActionRequestStart(b)
+	fb.GameActionRequestAddGameId(b, gid)
+	b.Finish(fb.GameActionRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// stateReq builds a StateRequest payload. includeAlphabet asks the backend to embed
+// the variant alphabet table (the driver sets it once per variant).
+func stateReq(gameID string, includeAlphabet bool) []byte {
+	b := flatbuffers.NewBuilder(64)
+	gid := b.CreateString(gameID)
+	fb.StateRequestStart(b)
+	fb.StateRequestAddGameId(b, gid)
+	fb.StateRequestAddIncludeAlphabet(b, includeAlphabet)
+	b.Finish(fb.StateRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// submitPlay builds a SubmitPlayRequest payload. dir is "H" or "V"; tiles are the
+// newly-placed tiles in main-word order.
+func submitPlay(gameID, dir string, tiles []PlayTile) []byte {
+	b := flatbuffers.NewBuilder(256)
+	gid := b.CreateString(gameID)
+	d := b.CreateString(dir)
+	offs := make([]flatbuffers.UOffsetT, len(tiles))
+	for i, t := range tiles {
+		fb.PlayTileStart(b)
+		fb.PlayTileAddRow(b, int32(t.Row))
+		fb.PlayTileAddCol(b, int32(t.Col))
+		fb.PlayTileAddLetter(b, t.Letter)
+		fb.PlayTileAddBlank(b, t.Blank)
+		offs[i] = fb.PlayTileEnd(b)
+	}
+	fb.SubmitPlayRequestStartTilesVector(b, len(offs))
+	for i := len(offs) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(offs[i])
+	}
+	tilesVec := b.EndVector(len(offs))
+	fb.SubmitPlayRequestStart(b)
+	fb.SubmitPlayRequestAddGameId(b, gid)
+	fb.SubmitPlayRequestAddDir(b, d)
+	fb.SubmitPlayRequestAddTiles(b, tilesVec)
+	b.Finish(fb.SubmitPlayRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// exchange builds an ExchangeRequest payload swapping the listed rack tiles (alphabet
+// indices; 255 a blank).
+func exchange(gameID string, tiles []byte) []byte {
+	b := flatbuffers.NewBuilder(64)
+	gid := b.CreateString(gameID)
+	vec := b.CreateByteVector(tiles)
+	fb.ExchangeRequestStart(b)
+	fb.ExchangeRequestAddGameId(b, gid)
+	fb.ExchangeRequestAddTiles(b, vec)
+	b.Finish(fb.ExchangeRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// checkWord builds a CheckWordRequest payload (alphabet indices for the word).
+func checkWord(gameID string, word []byte) []byte {
+	b := flatbuffers.NewBuilder(64)
+	gid := b.CreateString(gameID)
+	vec := b.CreateByteVector(word)
+	fb.CheckWordRequestStart(b)
+	fb.CheckWordRequestAddGameId(b, gid)
+	fb.CheckWordRequestAddWord(b, vec)
+	b.Finish(fb.CheckWordRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// chatPost builds a ChatPostRequest payload.
+func chatPost(gameID, body string) []byte {
+	b := flatbuffers.NewBuilder(128)
+	gid := b.CreateString(gameID)
+	bd := b.CreateString(body)
+	fb.ChatPostRequestStart(b)
+	fb.ChatPostRequestAddGameId(b, gid)
+	fb.ChatPostRequestAddBody(b, bd)
+	b.Finish(fb.ChatPostRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// draftSave builds a DraftRequest payload carrying the opaque composition JSON.
+func draftSave(gameID, jsonStr string) []byte {
+	b := flatbuffers.NewBuilder(128)
+	gid := b.CreateString(gameID)
+	j := b.CreateString(jsonStr)
+	fb.DraftRequestStart(b)
+	fb.DraftRequestAddGameId(b, gid)
+	fb.DraftRequestAddJson(b, j)
+	b.Finish(fb.DraftRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// updateProfile builds an UpdateProfileRequest payload. It resends the marker display
+// name and sane defaults so the account stays findable by the seeder's Cleanup.
+func updateProfile(displayName, lang string) []byte {
+	b := flatbuffers.NewBuilder(192)
+	name := b.CreateString(displayName)
+	pl := b.CreateString(lang)
+	tz := b.CreateString("UTC")
+	as := b.CreateString("00:00")
+	ae := b.CreateString("07:00")
+	fb.UpdateProfileRequestStart(b)
+	fb.UpdateProfileRequestAddDisplayName(b, name)
+	fb.UpdateProfileRequestAddPreferredLanguage(b, pl)
+	fb.UpdateProfileRequestAddTimeZone(b, tz)
+	fb.UpdateProfileRequestAddAwayStart(b, as)
+	fb.UpdateProfileRequestAddAwayEnd(b, ae)
+	fb.UpdateProfileRequestAddBlockChat(b, false)
+	fb.UpdateProfileRequestAddBlockFriendRequests(b, false)
+	fb.UpdateProfileRequestAddNotificationsInAppOnly(b, true)
+	b.Finish(fb.UpdateProfileRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// createInvitation builds a CreateInvitationRequest payload. turnTimeoutSecs 0 asks
+// the backend for its default; dropoutTiles "remove" is the standard policy.
+func createInvitation(inviteeIDs []string, variant string, turnTimeoutSecs int) []byte {
+	b := flatbuffers.NewBuilder(256)
+	idOffs := make([]flatbuffers.UOffsetT, len(inviteeIDs))
+	for i, id := range inviteeIDs {
+		idOffs[i] = b.CreateString(id)
+	}
+	fb.CreateInvitationRequestStartInviteeIdsVector(b, len(idOffs))
+	for i := len(idOffs) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(idOffs[i])
+	}
+	ids := b.EndVector(len(idOffs))
+	variantOff := b.CreateString(variant)
+	dropout := b.CreateString("remove")
+	fb.CreateInvitationRequestStart(b)
+	fb.CreateInvitationRequestAddInviteeIds(b, ids)
+	fb.CreateInvitationRequestAddVariant(b, variantOff)
+	fb.CreateInvitationRequestAddTurnTimeoutSecs(b, int32(turnTimeoutSecs))
+	fb.CreateInvitationRequestAddHintsAllowed(b, true)
+	fb.CreateInvitationRequestAddHintsPerPlayer(b, 1)
+	fb.CreateInvitationRequestAddDropoutTiles(b, dropout)
+	b.Finish(fb.CreateInvitationRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// invitationAction builds an InvitationActionRequest payload (accept / decline /
+// cancel by id).
+func invitationAction(invitationID string) []byte {
+	b := flatbuffers.NewBuilder(64)
+	id := b.CreateString(invitationID)
+	fb.InvitationActionRequestStart(b)
+	fb.InvitationActionRequestAddInvitationId(b, id)
+	b.Finish(fb.InvitationActionRequestEnd(b))
+	return b.FinishedBytes()
+}
+
+// enqueueReq builds an EnqueueRequest payload (join the per-variant auto-match pool).
+func enqueueReq(variant string) []byte {
+	b := flatbuffers.NewBuilder(64)
+	v := b.CreateString(variant)
+	fb.EnqueueRequestStart(b)
+	fb.EnqueueRequestAddVariant(b, v)
+	b.Finish(fb.EnqueueRequestEnd(b))
+	return b.FinishedBytes()
+}