feat(forge): fondation multi-forges — connexions chiffrées, registre, client GitHub (P0/P1)

- Tables forge_connections/users/workspaces (+ membres, projets,
  états, audit) ; migration des anciennes bases avec portées
  (workspace_id, connection_id) par défaut (1, 1).
- Jetons chiffrés Fernet (KANBANTE_MASTER_KEY ou clé générée en
  base), jamais réaffichés.
- ForgeRegistry : connexion héritée créée depuis l'environnement,
  espace Principal, instantanés préfixés par connexion.
- GitHubClient complet (Bearer, pagination Link, étiquettes par
  nom, PATCH archived, branches via git/refs).
- Paramètres surchargeables en base (résolveur config, P3).

app/forge_crypto.py

@@ -0,0 +1,70 @@
+"""Encryption at rest for forge tokens (0.24.0, P0/P2).
+
+Forge access tokens are the keys to the kingdom: they are
+stored Fernet-encrypted (AES-128-CBC + HMAC) in
+forge_connections.token_encrypted and never shown again after
+entry (the admin UI only ever displays a mask).
+
+Master key resolution (decision D5):
+1. KANBANTE_MASTER_KEY (urlsafe base64, 32 bytes) — recommended;
+2. otherwise a random key generated ONCE and kept in kv_store
+   ("master_key_generated"): as safe as the database file
+   itself, zero configuration, but it travels with DB backups —
+   set the env var for anything stricter.
+"""
+
+import logging
+import os
+import secrets as _secrets
+
+from cryptography.fernet import Fernet
+
+from app import models
+
+logger = logging.getLogger("kanbante.crypto")
+
+_fernet: Fernet | None = None
+
+
+def _master_key() -> bytes:
+    env_key = os.getenv("KANBANTE_MASTER_KEY", "").strip()
+    if env_key:
+        return env_key.encode("utf-8")
+    stored = models.get_kv("master_key_generated")
+    if stored:
+        return stored.encode("utf-8")
+    generated = Fernet.generate_key()
+    models.set_kv("master_key_generated", generated.decode("utf-8"))
+    logger.warning(
+        "KANBANTE_MASTER_KEY not set: generated a master key and "
+        "stored it in the database (kv_store). Set the env var to "
+        "control it explicitly."
+    )
+    return generated
+
+
+def _get_fernet() -> Fernet:
+    global _fernet
+    if _fernet is None:
+        _fernet = Fernet(_master_key())
+    return _fernet
+
+
+def encrypt_token(plain: str) -> str:
+    if not plain:
+        return ""
+    return _get_fernet().encrypt(plain.encode("utf-8")).decode("utf-8")
+
+
+def decrypt_token(stored: str) -> str:
+    if not stored:
+        return ""
+    try:
+        return _get_fernet().decrypt(stored.encode("utf-8")).decode("utf-8")
+    except Exception as exc:
+        logger.error("forge token decryption failed: %s", exc)
+        return ""
+
+
+def mask_token(stored: str) -> str:
+    return "••••••••" if stored else "—"