feat: password change endpoint with full session rotation

Adds the pre-1.0 password-change story flagged by the audit. Browser
users and bot owners both go through PATCH /api/v1/auth/me/password
with the current + new password in the body.

Implementation in `api::auth::change_password`:
- CurrentUser-gated: 401 if unauthenticated.
- Verifies current_password against the stored argon2 hash. Wrong
  current → 401 unauthenticated, matching the login contract.
- new_password runs through the same `validate_password` used at
  registration (≥8 chars). Weak → 400 invalid_input.
- On success, wraps the swap in a single transaction:
  - UPDATE users.password_hash with a fresh argon2 hash.
  - DELETE every session for this user (signs out other devices —
    any cookie stolen before the change is dead now).
  - INSERT a new session and mint a fresh cookie so the caller stays
    logged in.
- 204 + Set-Cookie on success.

Bot tokens (api_tokens) are intentionally left alone. They're explicit
opt-in credentials that the user can already audit and revoke
individually via DELETE /auth/tokens/{id}; rotating them on every
password change would surprise CI scripts.

Repo refactor: `repo::session::create` accepts `impl PgExecutor<'_>`
(same pattern feat/uploads used for chapters), and a new
`session::delete_all_for_user` covers the "sign out everywhere"
write. The existing `delete_by_token_hash` (used by logout) is
unchanged.

Coverage in tests/api_auth.rs (4 cases):
- change_password_rotates_sessions_and_swaps_credentials — happy path
  asserts the new cookie differs from the original, that both the
  original cookie AND a second-device cookie become invalid, that the
  new cookie keeps working, that login with the old password fails
  (401) and login with the new password succeeds.
- change_password_rejects_wrong_current_with_401 — wrong current
  password returns 401 unauthenticated.
- change_password_rejects_weak_new_password — new_password "short"
  returns 400 invalid_input.
- change_password_requires_authentication — no cookie returns 401.

README updated with the new endpoint in the auth table.

Lockstep version bump to 0.10.0.

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

README.md

@@ -96,6 +96,7 @@ Everything is namespaced under `/api/v1/`. `/api/*` outside the version prefix i
 | POST   | `/api/v1/auth/login`                                    | Same shape; rotates to a fresh session.                |
 | POST   | `/api/v1/auth/logout`                                   | Invalidates the session, clears the cookie.            |
 | GET    | `/api/v1/auth/me`                                       | Current user, or 401 if anonymous.                     |
+| PATCH  | `/api/v1/auth/me/password`                              | `{current_password,new_password}` → 401 on wrong current; on success deletes **all** of the user's sessions and mints a fresh one for the caller. Bot tokens stay (use DELETE /tokens/{id} to revoke). |
 | POST   | `/api/v1/auth/tokens`                                   | `{name}` → bot bearer token (returned once).           |
 | DELETE | `/api/v1/auth/tokens/{id}`                              | Revoke a bot token (owner-only).                       |
 | POST   | `/api/v1/mangas`                                        | Multipart: `metadata` JSON + optional `cover` image.   |