Throttle the change-password route and make epoch bumps atomic

Two session-management bugs from the audit:

- /api/password recorded failed attempts against the login budget but never
  checked it, so a borrowed session could guess the current password without
  limit while still locking the owner out of /api/login. Verified: thirteen
  consecutive wrong guesses all returned 403 and none returned 429. It now
  spends from the same budget it was topping up.

- bump_epoch read the epoch and wrote it back without the write lock, so
  concurrent revocations lost increments and sessions that should have been cut
  off survived. Verified: twenty concurrent bumps advanced the counter from 2 to
  6, and twenty concurrent "sign out other devices" calls left four sessions
  authenticated. It takes BEGIN IMMEDIATE now; set_password hashes before
  locking, so scrypt doesn't serialise unrelated writes.

Removing either fix makes its test fail with exactly that symptom.

README corrections: deployment is rsync, not git pull — gitea on .8 cannot
serve a clone to .8 itself, which the deploy section now documents — and the
concurrency check count was understated.

Checks go from 182 to 197.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/auth.py

@@ -115,7 +115,11 @@ def check_password(conn, candidate: str) -> bool:
 
 def set_password(conn, new_password: str):
     """Store a new password and invalidate every outstanding session."""
-    db.set_setting(conn, PASSWORD_KEY, hash_password(new_password))
+    # Hash before taking the lock: scrypt is deliberately slow, and holding
+    # SQLite's write lock across it would serialise unrelated writes.
+    hashed = hash_password(new_password)
+    db.begin_immediate(conn)
+    db.set_setting(conn, PASSWORD_KEY, hashed)
     bump_epoch(conn)
 
 
@@ -141,6 +145,10 @@ def current_epoch(conn) -> int:
 
 
 def bump_epoch(conn) -> int:
+    # Read-modify-write, so it needs the write lock across both halves. Without
+    # it, concurrent "sign out other devices" calls read the same epoch and
+    # overwrite each other, and sessions that should have been cut off survive.
+    db.begin_immediate(conn)
     epoch = current_epoch(conn) + 1
     db.set_setting(conn, EPOCH_KEY, epoch)
     return epoch