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>

backend/src/api/auth.rs

@@ -7,7 +7,7 @@
 use axum::extract::{Path, State};
 use axum::http::StatusCode;
 use axum::response::IntoResponse;
-use axum::routing::{delete, get, post};
+use axum::routing::{delete, get, patch, post};
 use axum::{Json, Router};
 use axum_extra::extract::cookie::{Cookie, CookieJar, SameSite};
 use chrono::{Duration, Utc};
@@ -29,6 +29,7 @@ pub fn routes() -> Router<AppState> {
         .route("/auth/login", post(login))
         .route("/auth/logout", post(logout))
         .route("/auth/me", get(me))
+        .route("/auth/me/password", patch(change_password))
         .route("/auth/tokens", post(create_token))
         .route("/auth/tokens/:id", delete(delete_token))
 }
@@ -49,6 +50,12 @@ pub struct CreateTokenInput {
     pub name: String,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct ChangePassword {
+    pub current_password: String,
+    pub new_password: String,
+}
+
 #[derive(Debug, Serialize)]
 pub struct CreatedTokenResponse {
     #[serde(flatten)]
@@ -111,6 +118,49 @@ async fn me(CurrentUser(user): CurrentUser) -> AppResult<Json<AuthResponse>> {
     Ok(Json(AuthResponse { user }))
 }
 
+/// `PATCH /api/v1/auth/me/password` — change the current user's password.
+///
+/// Verifies `current_password` against the stored argon2 hash (401 on
+/// mismatch, matching the login contract). Validates `new_password`
+/// against the same min-length rule used at registration. On success,
+/// inside a single transaction:
+///   - UPDATE users.password_hash with the new argon2 hash
+///   - DELETE all existing sessions for this user (signs out other
+///     devices; the stolen-cookie attack surface is closed)
+///   - INSERT a fresh session and return it as a new cookie so the
+///     caller stays logged in
+///
+/// Bot tokens (`api_tokens`) are left alone: the user opted into them
+/// explicitly and can revoke individually via DELETE /auth/tokens/{id}.
+async fn change_password(
+    State(state): State<AppState>,
+    CurrentUser(user): CurrentUser,
+    jar: CookieJar,
+    Json(input): Json<ChangePassword>,
+) -> AppResult<impl IntoResponse> {
+    if !verify_password(&input.current_password, &user.password_hash) {
+        return Err(AppError::Unauthenticated);
+    }
+    validate_password(&input.new_password)?;
+
+    let new_hash = hash_password(&input.new_password)?;
+
+    let mut tx = state.db.begin().await?;
+    sqlx::query("UPDATE users SET password_hash = $1 WHERE id = $2")
+        .bind(&new_hash)
+        .bind(user.id)
+        .execute(&mut *tx)
+        .await?;
+    repo::session::delete_all_for_user(&mut *tx, user.id).await?;
+    let (raw, hash) = generate_token();
+    let expires_at = Utc::now() + Duration::days(state.auth.session_ttl_days);
+    repo::session::create(&mut *tx, user.id, &hash, expires_at).await?;
+    tx.commit().await?;
+
+    let jar = jar.add(build_session_cookie(raw, &state.auth));
+    Ok((StatusCode::NO_CONTENT, jar))
+}
+
 async fn create_token(
     State(state): State<AppState>,
     CurrentUser(user): CurrentUser,