feat(i18n): add scoped i18n foundation with one migrated page (#2097)

Co-authored-by: Kandev Agent <agent@kandev.dev>

apps/backend/AGENTS.md

@@ -275,6 +275,27 @@ Every long-running goroutine must have a single owner with explicit start and st
 
 You may still list the column in the `CREATE TABLE` so fresh DBs get it inline, but the migration is the source of truth for evolution and must stand alone. New columns also need: the struct field in `models/`, the DTO field + `ToAPI` in `pkg/api/v1/`, and every `CreateX`/`UpdateX`/bulk write in the repo that should set it.
 
+## Internationalization
+
+`internal/i18n` covers only what Go renders **directly to a browser**, which today
+is just the SPA-unavailable error pages. Everything else stays English by design —
+the ~1,100 `http.Error` / `gin.H{"error": …}` strings are diagnostics the SPA maps
+onto its own translated copy, and log lines, agent/ACP output, and CLI output are
+not display copy. The shared-task artifacts (`share.html`, gist README and
+description) are server-rendered and in scope for localization, but not done yet.
+
+- `i18n.T(locale, key)` for lookup, `i18n.FromRequest(r)` at the HTTP boundary.
+- The package has no placeholder interpolation and no context-carried locale.
+  Both are needed for the share artifacts; add them with that work rather than
+  ahead of it.
+- Catalogs are embedded JSON (`internal/i18n/locales/<locale>.json`). `pseudo` is
+  **generated** — run `pnpm run i18n:pseudo` from `apps/web`, which writes both
+  the frontend and backend catalogs. A per-package test fails if an `en` key is
+  missing from `pseudo`.
+- **For new user-facing output, prefer returning a stable error code** the
+  frontend translates. Reach for `i18n.T` only when Go itself writes the bytes a
+  user reads.
+
 ## Code-quality limits
 
 Enforced by `apps/backend/.golangci.yml` (errors on new code only):