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/snapshots.py

@@ -0,0 +1,116 @@
+"""Background snapshot refresher — chantier B (0.22.0).
+
+The expensive part of Kanbante is the fan-out of Gitea list
+calls behind the portfolio pages (repo index, then issues and
+milestones for every active project). Since 0.19.0 those calls
+are parallel and stale-while-revalidate, which fixed repeat
+loads, but the FIRST page after a restart still paid seconds.
+
+The refresher pays that cost instead of the user: every
+KANBANTE_SNAPSHOT_INTERVAL seconds it force-refreshes the hot
+keys (repo index, starred, issues + milestones of active
+repos). Every fetch write-throughs to the SQLite `snapshots`
+table, and the client cache is hydrated from that table at
+startup — so pages read local data from the very first request
+and only ever wait on Gitea when no snapshot exists yet.
+
+Freshness contract, shown in the UI: Gitea-side data may lag
+by up to one interval; a Gitea webhook refreshes the touched
+repo immediately (see refresh_repo_snapshots); anything the
+user does locally (board journal) is always instant.
+"""
+
+import asyncio
+import datetime
+import logging
+
+from app import models
+from app.config import get_settings
+
+logger = logging.getLogger("kanbante.snapshots")
+
+_REFRESH_CONCURRENCY = 8
+
+
+async def refresh_repo_snapshots(gitea, owner: str, repo: str) -> None:
+    """Refresh one repo's hot keys (webhooks call this).
+
+    Both issue states are refreshed: most pages read "all", but
+    Focus/Triage/Mes issues read "open" — a snapshot set missing
+    either leaves a slow first hit on the table.
+    """
+    try:
+        await gitea.refresh_repo_issues(owner, repo, "all")
+        await gitea.refresh_repo_issues(owner, repo, "open")
+        await gitea.refresh_milestones(owner, repo)
+    except Exception as exc:
+        logger.debug("repo snapshot refresh failed for %s/%s: %s", owner, repo, exc)
+
+
+async def refresh_snapshots(gitea, classify_repos) -> int:
+    """One full refresh cycle. Returns the active-repo count."""
+    repos = await gitea.refresh_search_repos()
+    try:
+        await gitea.refresh_starred()
+    except Exception as exc:
+        logger.debug("starred refresh failed: %s", exc)
+    active, _inactive, _archived = classify_repos(repos)
+    semaphore = asyncio.Semaphore(_REFRESH_CONCURRENCY)
+
+    async def _one(repo: dict) -> None:
+        owner, _, name = repo["full_name"].partition("/")
+        async with semaphore:
+            await refresh_repo_snapshots(gitea, owner, name)
+
+    await asyncio.gather(*(_one(repo) for repo in active))
+    logger.info("snapshot refresh complete (%d active repos)", len(active))
+    return len(active)
+
+
+async def snapshot_loop(gitea, classify_repos) -> None:
+    """Periodic refresher, started by the app lifespan.
+
+    Sleeps briefly first: hydration already serves the pages,
+    the loop only has to keep the snapshots warm.
+    """
+    interval = get_settings().snapshot_interval
+    if interval <= 0:
+        logger.info("snapshot refresher disabled (interval <= 0)")
+        return
+    await asyncio.sleep(10)
+    while True:
+        try:
+            await refresh_snapshots(gitea, classify_repos)
+        except Exception as exc:
+            logger.warning("snapshot refresh cycle failed: %s", exc)
+        await asyncio.sleep(interval)
+
+
+def freshness_label() -> str | None:
+    """« à l'instant » / « il y a N min » from the freshest snapshot.
+
+    Honest freshness display (chantier B contract): the sidebar
+    shows how old the Gitea data on screen can be.
+    """
+    try:
+        latest = models.latest_snapshot_at()
+    except Exception:
+        return None
+    if not latest:
+        return None
+    try:
+        fetched = datetime.datetime.strptime(
+            latest, "%Y-%m-%d %H:%M:%S"
+        ).replace(tzinfo=datetime.timezone.utc)
+    except ValueError:
+        return None
+    age = (
+        datetime.datetime.now(datetime.timezone.utc) - fetched
+    ).total_seconds()
+    if age < 90:
+        return "à l'instant"
+    if age < 3600:
+        return f"il y a {int(age // 60)} min"
+    if age < 86400:
+        return f"il y a {int(age // 3600)} h"
+    return f"il y a {int(age // 86400)} j"