diplomail (Stage A): add in-game personal mail subsystem

Phase 28 of ui/PLAN.md needs a persistent player-to-player mail
channel; the existing `mail` package is a transactional email
outbox and the `notification` catalog is one-way platform events.
Stage A lands the schema (diplomail_messages / _recipients /
_translations), a single-recipient personal send/read/delete
service path, a `diplomail.message.received` push kind plumbed
through the notification pipeline, and an unread-counts endpoint
that drives the lobby badge. Admin / system mail, lifecycle hooks,
paid-tier broadcast, multi-game broadcast, bulk purge and language
detection / translation cache come in stages B–D.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/diplomail/errors.go

@@ -0,0 +1,32 @@
+package diplomail
+
+import "errors"
+
+// Sentinel errors surface common rejection reasons across the
+// diplomail package. Handlers map them to HTTP envelopes through
+// `respondDiplomailError` in `internal/server/handlers_user_mail.go`.
+//
+// Adding a new sentinel here is a deliberate API change: it appears in
+// the handler error map and may surface as a new wire `code` value.
+// Reuse the existing set when the behaviour overlaps.
+var (
+	// ErrInvalidInput reports request-level validation failures
+	// (empty body, body or subject over the configured byte limit,
+	// invalid UUID, non-UTF-8 bytes). Maps to 400 invalid_request.
+	ErrInvalidInput = errors.New("diplomail: invalid input")
+
+	// ErrNotFound reports that the requested message does not exist
+	// or is not visible to the caller. Maps to 404 not_found.
+	ErrNotFound = errors.New("diplomail: not found")
+
+	// ErrForbidden reports that the caller is authenticated but not
+	// authorised for the requested action (not an active member of
+	// the game; not a recipient of the message). Maps to 403
+	// forbidden.
+	ErrForbidden = errors.New("diplomail: forbidden")
+
+	// ErrConflict reports that the requested action conflicts with
+	// the current persisted state (e.g. soft-deleting a message
+	// that has not been marked read yet). Maps to 409 conflict.
+	ErrConflict = errors.New("diplomail: conflict")
+)