feat(auth): sessions module + permissions matrix + rate limiter

SQLite drops tzinfo on DateTime roundtrip — normalize on read/write
so callers see tz-aware datetimes.

backend/src/cyclone/auth/permissions.py

@@ -0,0 +1,73 @@
+"""Role enum + PERMISSIONS matrix."""
+
+from __future__ import annotations
+
+from enum import Enum
+
+
+class Role(str, Enum):
+    ADMIN = "admin"
+    USER = "user"
+    VIEWER = "viewer"
+
+
+ALL_ROLES = {Role.ADMIN, Role.USER, Role.VIEWER}
+WRITE_ROLES = {Role.ADMIN, Role.USER}
+ADMIN_ONLY = {Role.ADMIN}
+
+
+# (method, path-prefix) → allowed roles.
+# Endpoints not in this matrix default to DENY (fail-closed).
+PERMISSIONS: dict[tuple[str, str], set[Role]] = {
+    # Public paths.
+    ("GET", "/api/healthz"): set(),
+    ("POST", "/api/auth/login"): set(),
+
+    # Auth surface.
+    ("POST", "/api/auth/logout"): ALL_ROLES,
+    ("GET", "/api/auth/me"): ALL_ROLES,
+
+    # Admin-only user management.
+    ("GET", "/api/admin/users"): ADMIN_ONLY,
+    ("POST", "/api/admin/users"): ADMIN_ONLY,
+    ("PATCH", "/api/admin/users"): ADMIN_ONLY,
+    ("DELETE", "/api/admin/users"): ADMIN_ONLY,
+
+    # Read endpoints (all authenticated roles).
+    ("GET", "/api/claims"): ALL_ROLES,
+    ("GET", "/api/remittances"): ALL_ROLES,
+    ("GET", "/api/providers"): ALL_ROLES,
+    ("GET", "/api/batches"): ALL_ROLES,
+    ("GET", "/api/dashboard/summary"): ALL_ROLES,
+    ("GET", "/api/activity"): ALL_ROLES,
+    ("GET", "/api/inbox/lanes"): ALL_ROLES,
+    ("GET", "/api/reconcile"): ALL_ROLES,
+    ("GET", "/api/audit-log"): ADMIN_ONLY,
+
+    # Write endpoints (admin + user, no viewer).
+    ("POST", "/api/parse-837"): WRITE_ROLES,
+    ("POST", "/api/parse-835"): WRITE_ROLES,
+    ("POST", "/api/inbox"): WRITE_ROLES,
+    ("POST", "/api/reconcile"): WRITE_ROLES,
+    ("POST", "/api/resubmit"): WRITE_ROLES,
+    ("POST", "/api/acks"): WRITE_ROLES,
+
+    # CSV export — read-only.
+    ("GET", "/api/export.csv"): ALL_ROLES,
+}
+
+
+def allowed_roles(method: str, path: str) -> set[Role] | None:
+    """Return the set of roles allowed to call (method, path), or None if denied.
+
+    Uses longest-prefix match on path; falls back to DENY (None) if no entry matches.
+    """
+    candidates = [
+        (len(prefix), roles)
+        for (m, prefix), roles in PERMISSIONS.items()
+        if m == method and (path == prefix or path.startswith(prefix.rstrip("/") + "/"))
+    ]
+    if not candidates:
+        return None
+    candidates.sort(key=lambda x: -x[0])
+    return candidates[0][1]

backend/src/cyclone/auth/rate_limit.py

@@ -0,0 +1,34 @@
+"""Per-username login rate limiter (in-memory, per-process)."""
+
+from __future__ import annotations
+
+import time
+from threading import Lock
+
+WINDOW_SECONDS = 300
+MAX_FAILS = 5
+
+_FAILS: dict[str, list[float]] = {}
+_LOCK = Lock()
+
+
+def check(username: str) -> int:
+    """Return retry-after seconds, or 0 if allowed."""
+    now = time.monotonic()
+    with _LOCK:
+        fails = [t for t in _FAILS.get(username, []) if now - t < WINDOW_SECONDS]
+        _FAILS[username] = fails
+        if len(fails) >= MAX_FAILS:
+            return int(WINDOW_SECONDS - (now - fails[0]))
+    return 0
+
+
+def record_failure(username: str) -> None:
+    now = time.monotonic()
+    with _LOCK:
+        _FAILS.setdefault(username, []).append(now)
+
+
+def reset(username: str) -> None:
+    with _LOCK:
+        _FAILS.pop(username, None)

backend/src/cyclone/auth/sessions.py

@@ -0,0 +1,60 @@
+"""Session create/validate/expire/touch."""
+
+from __future__ import annotations
+
+import secrets
+from datetime import datetime, timedelta, timezone
+
+from sqlalchemy import select
+
+from cyclone.db import Session
+
+SESSION_LIFETIME = timedelta(hours=24)
+
+
+def create(db, *, user_id: int) -> tuple[str, Session]:
+    sid = secrets.token_urlsafe(32)
+    now = datetime.now(timezone.utc)
+    expires_at = now + SESSION_LIFETIME
+    sess = Session(
+        id=sid,
+        user_id=user_id,
+        expires_at=expires_at,
+        created_at=now,
+    )
+    db.add(sess)
+    db.commit()
+    db.refresh(sess)
+    # SQLite strips tzinfo on roundtrip; restore it so callers don't have to.
+    sess.expires_at = expires_at
+    return sid, sess
+
+
+def get_valid(db, sid: str) -> Session | None:
+    sess = db.execute(
+        select(Session).where(Session.id == sid)
+    ).scalar_one_or_none()
+    if sess is None:
+        return None
+    # SQLite drops tzinfo on roundtrip; normalize to UTC before comparing.
+    if sess.expires_at.tzinfo is None:
+        sess.expires_at = sess.expires_at.replace(tzinfo=timezone.utc)
+    if sess.expires_at <= datetime.now(timezone.utc):
+        return None
+    return sess
+
+
+def delete(db, sid: str) -> None:
+    sess = db.get(Session, sid)
+    if sess is None:
+        return
+    db.delete(sess)
+    db.commit()
+
+
+def touch(db, sid: str) -> None:
+    sess = db.get(Session, sid)
+    if sess is None:
+        return
+    sess.expires_at = datetime.now(timezone.utc) + SESSION_LIFETIME
+    db.commit()