feat(auth): add session policy settings + account columns + migration

First commit in the session-expiration-policy series (see
docs/plans/2026-05-13-session-expiration-policy.md). No behavior change
yet — this lays the schema + settings groundwork only.

- Settings: SESSION_IDLE_MINUTES_DEFAULT=4320 (3d),
  SESSION_ABSOLUTE_MINUTES_DEFAULT=20160 (14d), plus MIN/MAX bounds
  so account overrides have envelopes (15min..30d idle, 1h..90d
  absolute).
- accounts table: nullable session_idle_minutes and
  session_absolute_minutes columns (NULL = use system default), plus
  a CHECK constraint that rejects idle > absolute when both are set.
  Partial-override validation lives at the app layer because the DB
  cannot read Settings.

Subsequent commits will: distinguish idle vs invalid-token expiry on
the wire, embed auth_time/idle_max/abs_max in refresh JWTs, enforce
the absolute cap in /auth/refresh, add the owner-only policy +
bulk-revoke endpoints, and surface everything in an AccountSecurity
settings page with a session-expiry toast.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/core/config.py

@@ -69,6 +69,19 @@ class Settings(BaseSettings):
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 5
     REFRESH_TOKEN_EXPIRE_DAYS: int = 7
 
+    # Session policy — see docs/plans/2026-05-13-session-expiration-policy.md
+    # Refresh tokens enforce two windows: idle (between rotations) and absolute
+    # (from original login). Defaults can be overridden per-account, bounded by
+    # the MIN/MAX values below. Values are minutes everywhere except inside the
+    # refresh JWT, where idle_max/abs_max are stored as seconds for direct
+    # Unix-time math.
+    SESSION_IDLE_MINUTES_DEFAULT: int = 4320      # 3 days
+    SESSION_ABSOLUTE_MINUTES_DEFAULT: int = 20160  # 14 days
+    SESSION_IDLE_MINUTES_MIN: int = 15
+    SESSION_IDLE_MINUTES_MAX: int = 43200          # 30 days
+    SESSION_ABSOLUTE_MINUTES_MIN: int = 60         # 1 hour
+    SESSION_ABSOLUTE_MINUTES_MAX: int = 129600     # 90 days
+
     # Security
     BCRYPT_ROUNDS: int = 12