feat(service-storage-harness): slice 1 — harness + qbit store + client + integration

ServiceDataHarness (services/service_data.py): lifecycle-only registry of
per-concern storage — DB provisioning, idempotent migrations (ALTER TABLE
duplicate-column-name caught per-statement), cascade_delete(service_id).
QbittorrentSampleStore: append/window/prune (MAX_SAMPLES=120) in
qbittorrent.db. QbittorrentClient: cookie-login Web API client (403 re-login,
/sync/maindata). Integration registered with 3 widget kinds (totals/active/
speed). Harness initialized in main.py lifespan.

Backend: 314 pytest pass (21 new), ruff clean.

backend/src/media_library_viewer_api/services/service_data.py

@@ -0,0 +1,153 @@
+"""Lifecycle-only storage harness for service-owned persistent data.
+
+This module owns the *lifecycle* of per-concern SQLite databases: provisioning,
+schema migrations, and cascade-delete when a service instance is removed. It
+does **not** own data operations — each integration implements its own Store
+with bespoke operations (``append``/``window``, ``replace_items``/``query``,
+etc.). This keeps the general interface narrow (lifecycle) and the specific
+interfaces rich (per-integration operations).
+
+Each storage *concern* is registered with a :class:`StorageConcern` dataclass
+declaring its DB filename, ordered migration statements, owned tables, and the
+column used for service scoping.
+"""
+
+from __future__ import annotations
+
+import logging
+import os
+import sqlite3
+from dataclasses import dataclass, field
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass(frozen=True)
+class StorageConcern:
+    """A per-integration storage namespace registered with the harness."""
+
+    concern_key: str  # e.g. "qbittorrent", "media_index"
+    db_filename: str  # e.g. "qbittorrent.db", "media_index.sqlite"
+    migrations: list[str]  # ordered CREATE/ALTER statements (idempotent via IF NOT EXISTS or ALTER-catch)
+    tables: list[str] = field(default_factory=list)  # tables owned by this concern (for cascade)
+    service_id_column: str = "service_id"
+
+
+class ServiceDataHarness:
+    """Lifecycle-only registry of per-concern storage.
+
+    Owns:
+      * DB provisioning (per-concern SQLite files under ``base_dir``).
+      * Schema migrations (run on first access via :meth:`run_migrations`).
+      * ``service_id`` cascade-delete when a service instance is removed.
+
+    Does **not** own:
+      * Data operations — each store keeps bespoke append/window/query/etc.
+      * A generic value table or generic CRUD layer.
+    """
+
+    def __init__(self, base_dir: Path | str) -> None:
+        self._base_dir = Path(base_dir)
+        self._concerns: dict[str, StorageConcern] = {}
+
+    @property
+    def base_dir(self) -> Path:
+        return self._base_dir
+
+    def register(self, concern: StorageConcern) -> None:
+        """Register a storage concern. Called at startup / on first access."""
+        self._concerns[concern.concern_key] = concern
+
+    def db_path(self, concern_key: str) -> Path:
+        """Return the absolute path to a concern's DB file."""
+        concern = self._concerns[concern_key]
+        return self._base_dir / concern.db_filename
+
+    def connect(self, concern_key: str) -> sqlite3.Connection:
+        """Open a WAL-mode connection to a concern's DB."""
+        path = self.db_path(concern_key)
+        path.parent.mkdir(parents=True, exist_ok=True)
+        conn = sqlite3.connect(path, timeout=30)
+        conn.row_factory = sqlite3.Row
+        conn.execute("PRAGMA journal_mode=WAL")
+        conn.execute("PRAGMA busy_timeout=30000")
+        return conn
+
+    def run_migrations(self) -> None:
+        """Run pending migrations for every registered concern.
+
+        Each migration string is split into individual statements (by ``;``)
+        and executed individually. ``ALTER TABLE ... ADD COLUMN`` statements
+        that fail with "duplicate column name" are silently skipped, making
+        migrations idempotent across re-runs and fresh installs where
+        ``init_schema`` may have already created the column.
+        """
+        for concern in self._concerns.values():
+            path = self.db_path(concern.concern_key)
+            path.parent.mkdir(parents=True, exist_ok=True)
+            conn = sqlite3.connect(path, timeout=30)
+            try:
+                conn.execute("PRAGMA journal_mode=WAL")
+                for migration_sql in concern.migrations:
+                    statements = [s.strip() for s in migration_sql.split(";") if s.strip()]
+                    for stmt in statements:
+                        try:
+                            conn.execute(stmt)
+                        except sqlite3.OperationalError as exc:
+                            if "duplicate column name" in str(exc).lower():
+                                logger.debug("Skipping already-applied migration: %s", stmt[:80])
+                            else:
+                                raise
+                conn.commit()
+            finally:
+                conn.close()
+
+    def cascade_delete(self, service_id: str) -> None:
+        """Delete all rows for a ``service_id`` across every concern's tables.
+
+        Called from :meth:`SettingsStore.delete_service` after the service row
+        is removed. Best-effort: callers wrap in try/except so a harness
+        failure does not block service deletion.
+        """
+        for concern in self._concerns.values():
+            col = concern.service_id_column
+            path = self.db_path(concern.concern_key)
+            if not path.exists():
+                continue
+            with sqlite3.connect(path, timeout=30) as conn:
+                for table in concern.tables:
+                    cols = {row[1] for row in conn.execute(f"PRAGMA table_info({table})").fetchall()}
+                    if col in cols:
+                        conn.execute(f"DELETE FROM {table} WHERE {col} = ?", (service_id,))
+
+
+# ---------------------------------------------------------------------------
+# Module-level singleton
+# ---------------------------------------------------------------------------
+
+_HARNESS: ServiceDataHarness | None = None
+
+
+def get_service_data_harness() -> ServiceDataHarness:
+    """Return the process-wide harness singleton, initializing it on first call.
+
+    Lazy registration of built-in concerns happens here (local imports avoid
+    circular dependencies). Migrations are run immediately after registration.
+    """
+    global _HARNESS
+    if _HARNESS is None:
+        base_dir = Path(os.environ.get("BACKEND_CACHE_DIR", ".cache/media_library_viewer"))
+        _HARNESS = ServiceDataHarness(base_dir)
+        # Register built-in concerns (lazy import avoids circular dependency).
+        from media_library_viewer_api.services.qbittorrent_store import QBITTORRENT_CONCERN
+
+        _HARNESS.register(QBITTORRENT_CONCERN)
+        _HARNESS.run_migrations()
+    return _HARNESS
+
+
+def reset_service_data_harness() -> None:
+    """Reset the singleton (for testing)."""
+    global _HARNESS
+    _HARNESS = None