feat(perf): chantier B — instantanés SQLite et rafraîchisseur d'arrière-plan

- Table snapshots + écriture traversante du cache client,
  hydratation au démarrage (SWR : servi + rafraîchi en fond).
- Boucle de rafraîchissement (120 s par défaut) : index, étoiles,
  issues all+open et jalons des projets actifs ; webhook =
  rafraîchissement immédiat du dépôt ; fraîcheur affichée.

app/models.py

@@ -11,6 +11,7 @@ and upgrades older databases in place (position column, notes
 updated_at, cleanup of rows carrying an unknown column name).
 """
 
+import json
 import os
 import sqlite3
 from contextlib import contextmanager
@@ -91,6 +92,12 @@ def init_db() -> None:
                 value TEXT NOT NULL
             );
 
+            CREATE TABLE IF NOT EXISTS snapshots (
+                key TEXT PRIMARY KEY,
+                payload TEXT NOT NULL,
+                updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+            );
+
             CREATE INDEX IF NOT EXISTS idx_events_repo
                 ON board_events(repo_full_name, issue_number, created_at);
             """
@@ -292,6 +299,48 @@ def set_kv(key: str, value: str) -> None:
         )
 
 
+# ── Snapshots (chantier B, 0.22.0) ─────────────────────────
+# Persisted copies of the Gitea list responses the pages read
+# (repos, issues, milestones, starred). The client cache is
+# hydrated from these rows at startup, so even the first page
+# after a restart renders from local data while the background
+# refresher re-fetches from Gitea.
+
+def save_snapshot(key: str, payload) -> None:
+    with get_db() as conn:
+        conn.execute(
+            "INSERT INTO snapshots (key, payload, updated_at) "
+            "VALUES (?, ?, datetime('now')) "
+            "ON CONFLICT(key) DO UPDATE SET payload = excluded.payload, "
+            "updated_at = excluded.updated_at",
+            (key, json.dumps(payload)),
+        )
+
+
+def load_snapshots() -> dict:
+    """All snapshots as {key: (payload, updated_at_utc_str)}."""
+    with get_db() as conn:
+        rows = conn.execute(
+            "SELECT key, payload, updated_at FROM snapshots"
+        ).fetchall()
+    out = {}
+    for row in rows:
+        try:
+            out[row["key"]] = (json.loads(row["payload"]), row["updated_at"])
+        except (ValueError, TypeError):
+            continue
+    return out
+
+
+def latest_snapshot_at() -> str | None:
+    """UTC timestamp (SQLite format) of the freshest snapshot."""
+    with get_db() as conn:
+        row = conn.execute(
+            "SELECT MAX(updated_at) AS latest FROM snapshots"
+        ).fetchone()
+    return row["latest"] if row else None
+
+
 # ── Event journal ──────────────────────────────────────────
 
 EVENT_TYPES = ("created", "moved", "closed", "reopened", "commented", "rule")