Add automatic IP blacklisting after 2 illegal contributions

Persistence: PostgreSQL table ip_abuse (ip_hash SHA-256, rejection_count,
timestamps, last_idea_id, blacklisted_at, expires_at). No raw IP stored.

Logic:
- First illegal contribution: recorded, tolerated (benefit of the doubt)
- Second illegal contribution: 30-day block (IP_BLACKLIST_DAYS, configurable)
- Counter is cumulative — valid contributions do not reset it
- Block check fires before all other validations in both submit routes

Backend:
- database.py: check_ip_blacklist, record_ip_rejection, get_ip_blacklist,
  remove_ip_blacklist; ip_abuse table created in init_db()
- app.py: _get_ip_hash() helper; blacklist check + record in submit_idea()
  and submit_consultation_idea(); admin routes GET/DELETE /api/admin/ip-blacklist

Admin panel: new "Blacklist" tab showing active entries with hash, rejection
count, trigger idea id, dates; "Lever" button for manual removal

Docs:
- SECURITE_ANTI_ABUS.md: section 8 describing the mechanism, RGPD basis,
  limits, and IP_BLACKLIST_DAYS env var
- privacy-policy.tsx: IP row updated to reflect hash storage + legal basis

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/flask-api/database.py

@@ -101,6 +101,19 @@ def init_db() -> None:
         )
         cur.execute("CREATE INDEX IF NOT EXISTS idx_ideas_consultation ON ideas(consultation_id)")
         cur.execute("CREATE INDEX IF NOT EXISTS idx_synthesis_consultation ON synthesis(consultation_id)")
+        # Table anti-abus — blacklist IP (RGPD art. 6(1)(f) — intérêt légitime)
+        cur.execute("""
+            CREATE TABLE IF NOT EXISTS ip_abuse (
+                ip_hash TEXT PRIMARY KEY,
+                rejection_count INTEGER NOT NULL DEFAULT 1,
+                first_rejected_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+                last_rejected_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+                last_idea_id INTEGER,
+                blacklisted_at TIMESTAMPTZ,
+                expires_at TIMESTAMPTZ
+            )
+        """)
+        cur.execute("CREATE INDEX IF NOT EXISTS idx_ip_abuse_expires ON ip_abuse(expires_at)")
     logger.info("Base de données initialisée.")
 
 
@@ -486,6 +499,74 @@ def get_consultation_contributions(
     return rows, total
 
 
+# ─── Anti-abus — Blacklist IP ─────────────────────────────────────────────────
+
+def check_ip_blacklist(ip_hash: str) -> bool:
+    """True si l'IP est blacklistée et que le blacklist n'a pas expiré."""
+    with db_cursor() as cur:
+        cur.execute(
+            "SELECT 1 FROM ip_abuse WHERE ip_hash = %s AND blacklisted_at IS NOT NULL AND expires_at > NOW()",
+            (ip_hash,),
+        )
+        return cur.fetchone() is not None
+
+
+def record_ip_rejection(ip_hash: str, idea_id: int, blacklist_days: int) -> bool:
+    """
+    Incrémente le compteur de rejets pour cette IP.
+    Déclenche le blacklist si rejection_count atteint 2.
+    Retourne True si le blacklist vient d'être activé.
+    """
+    with db_cursor() as cur:
+        cur.execute(
+            """
+            INSERT INTO ip_abuse (ip_hash, rejection_count, last_idea_id)
+            VALUES (%s, 1, %s)
+            ON CONFLICT (ip_hash) DO UPDATE
+                SET rejection_count = ip_abuse.rejection_count + 1,
+                    last_rejected_at = NOW(),
+                    last_idea_id = EXCLUDED.last_idea_id
+            RETURNING rejection_count, blacklisted_at
+            """,
+            (ip_hash, idea_id),
+        )
+        row = dict(cur.fetchone())
+        if row["rejection_count"] >= 2 and row["blacklisted_at"] is None:
+            cur.execute(
+                """
+                UPDATE ip_abuse
+                SET blacklisted_at = NOW(),
+                    expires_at = NOW() + (%s || ' days')::interval
+                WHERE ip_hash = %s
+                """,
+                (str(blacklist_days), ip_hash),
+            )
+            return True
+    return False
+
+
+def get_ip_blacklist() -> list[dict]:
+    """Liste des IPs blacklistées actives (non expirées), ordre anti-chronologique."""
+    with db_cursor() as cur:
+        cur.execute(
+            """
+            SELECT ip_hash, rejection_count, first_rejected_at,
+                   last_rejected_at, last_idea_id, blacklisted_at, expires_at
+            FROM ip_abuse
+            WHERE blacklisted_at IS NOT NULL AND expires_at > NOW()
+            ORDER BY blacklisted_at DESC
+            """
+        )
+        return [dict(row) for row in cur.fetchall()]
+
+
+def remove_ip_blacklist(ip_hash: str) -> bool:
+    """Lève le blacklist d'une IP (supprime l'entrée entièrement)."""
+    with db_cursor() as cur:
+        cur.execute("DELETE FROM ip_abuse WHERE ip_hash = %s RETURNING ip_hash", (ip_hash,))
+        return cur.fetchone() is not None
+
+
 def delete_consultation(consultation_id: int) -> bool:
     """Supprime une consultation et toutes ses données (idées, synthèse) dans la même transaction."""
     with db_cursor() as cur: