feat(forecast): prévisions au débit réel du journal (issue #7)

app/forecast.py: fermetures hebdo (8 sem.), débit moy/min/max sur
4 semaines complètes, ETA en fourchette, silence sans historique.
Dashboard (épuisement du backlog), bandeau milestone (alerte
échéance), page /forecast (sparkline SVG, verdicts), /api/metrics
étendu (weekly_closures, throughput_4w).

app/forecast.py

@@ -0,0 +1,119 @@
+"""Forecasts (roadmap #7): honest ETAs from the event journal.
+
+Throughput = issues closed per week, counted from board_events.
+Projections are deliberately given as a RANGE (best/worst weekly
+rate over the last 4 complete weeks) rather than a fake-precise
+date, and are withheld entirely when the journal has no closures
+to learn from — "not enough history" is an answer, too.
+"""
+
+from datetime import date, datetime, timedelta
+
+MONTHS_FR = [
+    "janvier", "février", "mars", "avril", "mai", "juin",
+    "juillet", "août", "septembre", "octobre", "novembre", "décembre",
+]
+MONTHS_FR_SHORT = [
+    "janv.", "févr.", "mars", "avr.", "mai", "juin",
+    "juil.", "août", "sept.", "oct.", "nov.", "déc.",
+]
+
+
+def french_date(d: date, short: bool = False) -> str:
+    months = MONTHS_FR_SHORT if short else MONTHS_FR
+    return f"{d.day} {months[d.month - 1]}"
+
+
+def _event_date(event: dict) -> date | None:
+    raw = str(event.get("created_at") or "")
+    try:
+        return datetime.strptime(raw[:10], "%Y-%m-%d").date()
+    except ValueError:
+        return None
+
+
+def weekly_closures(events: list[dict], weeks: int = 8,
+                    today: date | None = None) -> list[dict]:
+    """Closure counts per ISO week, oldest first, current week last."""
+    today = today or date.today()
+    current_monday = today - timedelta(days=today.weekday())
+    buckets: list[dict] = []
+    for i in range(weeks - 1, -1, -1):
+        start = current_monday - timedelta(weeks=i)
+        buckets.append({"start": start, "count": 0})
+    by_start = {b["start"]: b for b in buckets}
+    for event in events:
+        if event.get("event_type") != "closed":
+            continue
+        day = _event_date(event)
+        if day is None:
+            continue
+        monday = day - timedelta(days=day.weekday())
+        if monday in by_start:
+            by_start[monday]["count"] += 1
+    return buckets
+
+
+def throughput_stats(weekly: list[dict], last: int = 4) -> dict | None:
+    """avg/min/max closures per week over the last COMPLETE weeks.
+
+    The current (partial) week is excluded: on Monday morning it
+    would crush the average for no reason. Returns None when the
+    complete weeks contain no closure at all.
+    """
+    complete = weekly[:-1][-last:] if len(weekly) > 1 else []
+    if not complete:
+        return None
+    counts = [b["count"] for b in complete]
+    if sum(counts) == 0:
+        return None
+    return {
+        "avg": sum(counts) / len(counts),
+        "min": min(counts),
+        "max": max(counts),
+        "weeks": len(counts),
+    }
+
+
+def estimate(remaining: int, weekly: list[dict],
+             today: date | None = None) -> dict | None:
+    """Projected completion for `remaining` items at the real pace.
+
+    Returns {"done": True} when nothing remains, None when there is
+    no throughput to project from, else dates: central (avg rate),
+    earliest (best week), latest (worst nonzero week, if any).
+    """
+    today = today or date.today()
+    if remaining <= 0:
+        return {"done": True}
+    stats = throughput_stats(weekly)
+    if stats is None:
+        return None
+
+    def at_rate(rate: float) -> date:
+        days = remaining / rate * 7
+        return today + timedelta(days=max(1, round(days)))
+
+    result = {
+        "done": False,
+        "avg_per_week": round(stats["avg"], 1),
+        "central": at_rate(stats["avg"]),
+        "earliest": at_rate(stats["max"]) if stats["max"] > 0 else None,
+        "latest": at_rate(stats["min"]) if stats["min"] > 0 else None,
+    }
+    return result
+
+
+def eta_label(est: dict | None, done_text: str = "terminé") -> str | None:
+    """One-line French label for banners and cards (or None)."""
+    if est is None:
+        return None
+    if est.get("done"):
+        return f"✅ {done_text}"
+    label = f"à ce rythme ≈ {french_date(est['central'], short=True)}"
+    if est.get("earliest") and est.get("latest"):
+        label += (
+            f" (fourchette {french_date(est['earliest'], short=True)}"
+            f" – {french_date(est['latest'], short=True)})"
+        )
+    return label