v0.23.0: server-side sign-in state resume (roadmap #29)

Track each authenticated user's last-viewed route + light view state
server-side, and on next sign-in redirect them to that state, falling
back to the empty-state home only when there's no recorded state.

Backend:
- migration 022_user_session_state.sql: one row per user
  (user_id PK/FK, last_route, last_route_state JSON-as-TEXT,
  resume_enabled default 1, last_updated_at).
- PUT /api/me/last-state (require_user): upserts route + light state;
  no-ops when resume_enabled=0. Localized in the /me region.
- /api/auth/me payload now carries resume_enabled + last_route +
  decoded last_route_state (no extra round-trip).
- test_session_resume_vertical.py: auth-required, upsert/read-back,
  per-user isolation, resume_enabled=0 disable.

Frontend:
- lib/useLastState.js: debounced (~1s) route-change PUT for
  authenticated users; one-time resume redirect on sign-in, gated on
  identify having fired (preserves #21 Part C identify-then-track).
- api.js: putLastState() client call.
- App.jsx: import + call the hook; set identifyReady after identify.
  Header region untouched.

Per-user (not per-device); profile-settings opt-out toggle UI
deferred (column + default-on behavior ship now). Stored state is
route + light view state ONLY, never draft buffers — documented in
SPEC §6.8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/migrations/022_user_session_state.sql

@@ -0,0 +1,54 @@
+-- v0.23.0 / roadmap item #29: server-side sign-in state resume.
+--
+-- Track each authenticated user's last-viewed route + a small bag of
+-- "light" component state so that the *next* sign-in can land the user
+-- back where they left off, rather than always dropping them on the
+-- empty-state home view.
+--
+-- Storage shape (one row per user — per-user, NOT per-device, per the
+-- #29 "safe default"):
+--
+--   * `user_id` — PRIMARY KEY and FK into users(id) with cascade on
+--     delete. INTEGER to match users.id (INTEGER PRIMARY KEY
+--     AUTOINCREMENT). A deleted user automatically loses their stored
+--     resume state. One row per user means a later sign-in on any
+--     device resumes the most-recently-recorded route — the per-user
+--     model the roadmap asks for.
+--
+--   * `last_route` — the frontend pathname the user was last on
+--     (e.g. "/rfc/open-human-model"). TEXT, nullable until the first
+--     route-change POST lands. NEVER contains draft-buffer contents —
+--     it is a route only. See SPEC §6.2 "Sign-in state resume
+--     (privacy)".
+--
+--   * `last_route_state` — a JSON-encoded bag of *light* component
+--     state (scroll anchors, open-tab selection, filter chips, etc.).
+--     SQLite has no native JSONB; we store JSON as TEXT exactly as the
+--     rest of the app stores its JSON blobs (json.dumps / json.loads,
+--     cf. permission_events.details, actions.details). Nullable.
+--     PRIVACY INVARIANT: this column MUST NOT carry draft-buffer text,
+--     PR bodies, comment drafts, or any user-typed content — only
+--     ephemeral view state safe to replay. The PUT handler is the
+--     enforcement point; the column comment is the contract.
+--
+--   * `resume_enabled` — the per-user opt-out flag. 1 (default) means
+--     "resume me where I left off"; 0 means "always land on home". The
+--     PUT handler no-ops the upsert when this is 0, and the read path
+--     refuses to hand back a stored route when this is 0. A
+--     profile-settings toggle UI to flip this is a follow-up (the
+--     column + default-on behavior ship now); see CHANGELOG v0.23.0.
+--
+--   * `last_updated_at` — TEXT timestamp, app convention
+--     `datetime('now')`, matching device_trust.last_seen_at /
+--     users.last_seen_at. Refreshed on every successful upsert.
+--
+-- No new env vars. The debounce interval for the frontend route-change
+-- POST is a frontend constant (~1s), not a server knob.
+
+CREATE TABLE user_session_state (
+    user_id          INTEGER PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    last_route       TEXT,
+    last_route_state TEXT,                        -- JSON-encoded light state, nullable
+    resume_enabled   INTEGER NOT NULL DEFAULT 1,
+    last_updated_at  TEXT NOT NULL DEFAULT (datetime('now'))
+);