Sync 2: device-token bearer auth + linked-devices UI (M8)

Native clients (Tauri/Android) authenticate sync with a long-lived device
bearer token, alongside the existing web session cookie.

Backend:
- security.py: generate_token() (secrets.token_urlsafe) + hash_token()
  (SHA-256 — device tokens are already high-entropy, so no slow KDF; keeps
  per-request bearer auth cheap). Only the hash is stored.
- device_tokens table (migration 0016): id, user_id, token_hash (unique),
  name, created_at, last_used_at.
- login_required now accepts `Authorization: Bearer <token>` OR the session
  cookie. Session path stays DB-free (fast); bearer path looks up the token
  hash, sets g.user_id, and stamps last_used_at.
- Endpoints: POST /api/auth/device-login (public; email+password → token,
  the native first-link flow), POST /api/auth/devices (session/bearer →
  token, web "link a device"), GET /api/auth/devices (list), DELETE
  /api/auth/devices/<id> (revoke). All owner-scoped; token shown once.

Frontend:
- Per-user (not admin) /account view "Linked devices": create a token
  (one-time reveal + copy), list devices (name, linked/last-synced), revoke
  with confirm. Top-bar device icon for all users; devices Pinia store.

Tests (DB-free): token hash determinism + uniqueness; device endpoints
auth-guard (401 without auth, before DB); device-login input validation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/security.py

@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+import hashlib
+import secrets
+
 import bcrypt
 
 # bcrypt hashes at most 72 bytes and bcrypt>=4 raises on longer input, so we
@@ -16,3 +19,15 @@ def verify_password(password: str, password_hash: str) -> bool:
         return bcrypt.checkpw(password.encode("utf-8")[:_MAX_BCRYPT_BYTES], password_hash.encode("utf-8"))
     except (ValueError, TypeError):
         return False
+
+
+def generate_token() -> str:
+    """A high-entropy opaque device (bearer) token, URL-safe so it pastes cleanly."""
+    return secrets.token_urlsafe(32)
+
+
+def hash_token(token: str) -> str:
+    """One-way hash for device-token LOOKUP. A device token is already high-entropy
+    random, so a plain SHA-256 is enough (no slow KDF like passwords need) — which
+    keeps per-request bearer auth cheap. Only this hash is stored server-side."""
+    return hashlib.sha256(token.encode("utf-8")).hexdigest()