ui/phase-27: battle viewer (radial scene, playback, map markers)

Engine wire change: Report.battle switched from []uuid.UUID to
[]BattleSummary{id, planet, shots} so the map can place battle
markers without N extra fetches. FBS schema + generated Go/TS
regenerated; transcoder + report controller updated; openapi
adds the BattleSummary schema with a freeze test.

Backend gateway forwards engine GET /api/v1/battle/:turn/:uuid as
/api/v1/user/games/{game_id}/battles/{turn}/{battle_id} (handler
plus engineclient.FetchBattle, contract test stub, openapi spec).

UI:
- BattleViewer (lib/battle-player/) is a logically isolated SVG
  radial scene that consumes a BattleReport prop. Planet at the
  centre, races on the outer ring at equal angular spacing, race
  clusters by (race, className) with <class>:<numLeft> labels;
  observer groups (inBattle: false) are not drawn; eliminated
  races drop out and survivors re-distribute on the next frame.
- Shot line per frame: red on destroyed, green otherwise; erased
  on the next frame. Playback controls: play/pause + step ± +
  rewind + 1x/2x/4x speed (400/200/100 ms per frame).
- Page wrapper (lib/active-view/battle.svelte) loads BattleReport
  via api/battle-fetch.ts; synthetic-gameId prefix routes to a
  fixture loader, otherwise REST through the gateway. Always-
  visible <ol> text protocol satisfies the accessibility ask.
- section-battles.svelte links every battle UUID into the viewer.
- map/battle-markers.ts: yellow X cross of 2 LinePrim through the
  corners of the planet's circumscribed square (stroke width
  clamps from 1 px at 1 shot to 5 px at 100+ shots); bombing
  marker is a stroke-only ring (yellow when damaged, red when
  wiped). Wired into state-binding.ts; click handler dispatches
  battle clicks to the viewer and bombing clicks to the matching
  Reports row.
- i18n keys for the viewer in en + ru.

Docs: ui/docs/battle-viewer-ux.md, FUNCTIONAL.md §6.5 + ru
mirror, ui/PLAN.md Phase 27 decisions + deferred TODOs (push
event, richer class visuals, animated re-distribution).

Tests: Vitest unit (radial layout + timeline frame builder +
marker stroke formula + marker primitives), Playwright e2e for
the viewer (Reports link → viewer, playback step, not-found),
backend engineclient FetchBattle (200 / 404 / bad input), engine
openapi freezes (BattleReport, BattleReportGroup,
BattleActionReport, BattleSummary, Report.battle items).

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

backend/internal/server/handlers_user_games.go

@@ -243,6 +243,60 @@ func (h *UserGamesHandlers) Report() gin.HandlerFunc {
 	}
 }
 
+// Battle handles GET /api/v1/user/games/{game_id}/battles/{turn}/{battle_id}.
+// Forwards to the engine's `GET /api/v1/battle/:turn/:uuid`. Path
+// parameters are validated up-front to save a network hop. 404 from
+// the engine is forwarded as 404. The recipient race is resolved
+// from the runtime mapping but not forwarded — engine returns the
+// battle by id, visibility is enforced by the engine state.
+func (h *UserGamesHandlers) Battle() gin.HandlerFunc {
+	if h == nil || h.runtime == nil || h.engine == nil {
+		return handlers.NotImplemented("userGamesBattle")
+	}
+	return func(c *gin.Context) {
+		gameID, ok := parseGameIDParam(c)
+		if !ok {
+			return
+		}
+		turnRaw := c.Param("turn")
+		turn, err := strconv.Atoi(turnRaw)
+		if err != nil || turn < 0 {
+			httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, "turn must be a non-negative integer")
+			return
+		}
+		battleID := c.Param("battle_id")
+		if battleID == "" {
+			httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, "battle id is required")
+			return
+		}
+		userID, ok := userid.FromContext(c.Request.Context())
+		if !ok {
+			httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, "user id missing")
+			return
+		}
+		ctx := c.Request.Context()
+		if _, err := h.runtime.ResolvePlayerMapping(ctx, gameID, userID); err != nil {
+			respondGameProxyError(c, h.logger, "user games battle", ctx, err)
+			return
+		}
+		endpoint, err := h.runtime.EngineEndpoint(ctx, gameID)
+		if err != nil {
+			respondGameProxyError(c, h.logger, "user games battle", ctx, err)
+			return
+		}
+		body, status, err := h.engine.FetchBattle(ctx, endpoint, turn, battleID)
+		if err != nil {
+			respondEngineProxyError(c, h.logger, "user games battle", ctx, body, err)
+			return
+		}
+		if status == http.StatusNotFound {
+			httperr.Abort(c, http.StatusNotFound, httperr.CodeNotFound, "battle not found")
+			return
+		}
+		c.Data(http.StatusOK, "application/json", body)
+	}
+}
+
 // rebindActor decodes a JSON object from raw, sets `actor` to
 // raceName, and re-encodes. Backend never trusts the actor field
 // supplied by the client (per ARCHITECTURE.md §9).