single-user login screen

- server/credentials.py: PBKDF2 password + session secret, 0600 json,
  one-time setup command (server refuses to start without it)
- app.py: signed 30-day cookie (itsdangerous), middleware 401s API /
  redirects pages to /login?next=, 5-fail/15-min lockout, /logout
- static/login.html: dark-themed form, inline error display
- app.js: api() 401 -> redirect to /login preserving current URL
- docs: README quick start + features, SPEC auth section

server/credentials.py

@@ -0,0 +1,55 @@
+"""credentials.py — single-user password + session secret for ani-web.
+
+Run once:  .venv/bin/python -m server.credentials
+Stores a PBKDF2 password hash and a random session-signing secret in
+server/data/credentials.json (mode 0600). No env vars, no DB.
+"""
+import getpass
+import hashlib
+import json
+import os
+import secrets
+from pathlib import Path
+
+FILE = Path(__file__).parent / "data" / "credentials.json"
+ITER = 210_000  # OWASP 2023 minimum for PBKDF2-SHA256
+
+
+def set_password():
+    pw = getpass.getpass("Choose a password (min 8 chars): ")
+    if len(pw) < 8:
+        raise SystemExit("password too short")
+    if pw != getpass.getpass("Again: "):
+        raise SystemExit("passwords did not match")
+    salt = secrets.token_bytes(16)
+    data = {
+        "v": 1,
+        "salt": salt.hex(),
+        "hash": hashlib.pbkdf2_hmac("sha256", pw.encode(), salt, ITER).hex(),
+        "secret": secrets.token_hex(32),
+    }
+    FILE.parent.mkdir(exist_ok=True)
+    # ponytail: mode 0600 on create; won't tighten an existing file
+    fd = os.open(FILE, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    with os.fdopen(fd, "w") as f:
+        json.dump(data, f)
+    print(f"credentials saved to {FILE}")
+
+
+def load():
+    if not FILE.exists():
+        raise SystemExit(
+            "no credentials yet — set a password first:\n"
+            "  .venv/bin/python -m server.credentials")
+    return json.loads(FILE.read_text())
+
+
+def check(password: str) -> bool:
+    d = load()
+    h = hashlib.pbkdf2_hmac("sha256", password.encode(),
+                            bytes.fromhex(d["salt"]), d.get("iter", ITER))
+    return secrets.compare_digest(h.hex(), d["hash"])
+
+
+if __name__ == "__main__":
+    set_password()