feat: Stage 2 — authentication and setup flow

Backend (tasks 2.1–2.6, 2.10):
- settings_repo: get/set/delete/get_all CRUD for the key-value settings table
- session_repo: create/get/delete/delete_expired for session rows
- setup_service: bcrypt password hashing, one-time-only enforcement,
  run_setup() / is_setup_complete() / get_password_hash()
- auth_service: login() with bcrypt verify + token creation,
  validate_session() with expiry check, logout()
- setup router: GET /api/setup (status), POST /api/setup (201 / 409)
- auth router: POST /api/auth/login (token + HttpOnly cookie),
               POST /api/auth/logout (clears cookie, idempotent)
- SetupRedirectMiddleware: 307 → /api/setup for all API paths until setup done
- require_auth dependency: cookie or Bearer token → Session or 401
- conftest.py: manually bootstraps app.state.db for router tests
  (ASGITransport does not trigger ASGI lifespan)
- 85 tests pass; ruff 0 errors; mypy --strict 0 errors

Frontend (tasks 2.7–2.9):
- types/auth.ts, types/setup.ts, api/auth.ts, api/setup.ts
- AuthProvider: sessionStorage-backed context (isAuthenticated, login, logout)
- RequireAuth: guard component → /login?next=<path> when unauthenticated
- SetupPage: Fluent UI form, client-side validation, inline errors
- LoginPage: single password input, ?next= redirect after success
- DashboardPage: placeholder (full impl Stage 5)
- App.tsx: full route tree (/setup, /login, /, *)

backend/app/repositories/session_repo.py

@@ -0,0 +1,100 @@
+"""Session repository.
+
+Provides storage, retrieval, and deletion of session records in the
+``sessions`` table of the application SQLite database.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    import aiosqlite
+
+from app.models.auth import Session
+
+
+async def create_session(
+    db: aiosqlite.Connection,
+    token: str,
+    created_at: str,
+    expires_at: str,
+) -> Session:
+    """Insert a new session row and return the domain model.
+
+    Args:
+        db: Active aiosqlite connection.
+        token: Opaque random session token (hex string).
+        created_at: ISO 8601 UTC creation timestamp.
+        expires_at: ISO 8601 UTC expiry timestamp.
+
+    Returns:
+        The newly created :class:`~app.models.auth.Session`.
+    """
+    cursor = await db.execute(
+        "INSERT INTO sessions (token, created_at, expires_at) VALUES (?, ?, ?)",
+        (token, created_at, expires_at),
+    )
+    await db.commit()
+    return Session(
+        id=int(cursor.lastrowid) if cursor.lastrowid else 0,
+        token=token,
+        created_at=created_at,
+        expires_at=expires_at,
+    )
+
+
+async def get_session(db: aiosqlite.Connection, token: str) -> Session | None:
+    """Look up a session by its token.
+
+    Args:
+        db: Active aiosqlite connection.
+        token: The session token to retrieve.
+
+    Returns:
+        The :class:`~app.models.auth.Session` if found, else ``None``.
+    """
+    async with db.execute(
+        "SELECT id, token, created_at, expires_at FROM sessions WHERE token = ?",
+        (token,),
+    ) as cursor:
+        row = await cursor.fetchone()
+
+    if row is None:
+        return None
+
+    return Session(
+        id=int(row[0]),
+        token=str(row[1]),
+        created_at=str(row[2]),
+        expires_at=str(row[3]),
+    )
+
+
+async def delete_session(db: aiosqlite.Connection, token: str) -> None:
+    """Delete a session by token (logout / expiry clean-up).
+
+    Args:
+        db: Active aiosqlite connection.
+        token: The session token to remove.
+    """
+    await db.execute("DELETE FROM sessions WHERE token = ?", (token,))
+    await db.commit()
+
+
+async def delete_expired_sessions(db: aiosqlite.Connection, now_iso: str) -> int:
+    """Remove all sessions whose ``expires_at`` timestamp is in the past.
+
+    Args:
+        db: Active aiosqlite connection.
+        now_iso: Current UTC time as ISO 8601 string used as the cutoff.
+
+    Returns:
+        Number of rows deleted.
+    """
+    cursor = await db.execute(
+        "DELETE FROM sessions WHERE expires_at <= ?",
+        (now_iso,),
+    )
+    await db.commit()
+    return int(cursor.rowcount)