correctness batch: atomic writes, task refs, hmac, import-star, pickle

Per review §2:

- web/db.py: new _tx() context manager wraps multi-statement writers in
  BEGIN IMMEDIATE … COMMIT/ROLLBACK (our connections run in autocommit
  mode, so plain `with _lock:` doesn't give atomicity). partnership_accept
  (UPDATE + DELETE) and cleanup_retention (3 deletes/updates) now use it.
- Fire-and-forget tasks: add module-level _bg_tasks sets in web/app.py and
  web/enrichment.py. A _spawn() helper holds a strong ref until the task
  finishes so the GC can't drop it mid-flight (CPython's event loop only
  weakly references pending tasks).
- apply/main.py: require_api_key uses hmac.compare_digest, matching web's
  check. Also imports now use explicit names instead of `from settings *`.
- apply/language.py: replace `from settings import *` + `from paths import *`
  with explicit imports — this is the pattern that caused the LANGUAGE
  NameError earlier.
- alert/utils.py: pickle-based hash_any_object → deterministic JSON+sha256.
  Cheaper, portable across Python versions, no pickle attack surface.
- web/notifications.py: /fehler links repointed to /bewerbungen (the
  former page doesn't exist).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

alert/utils.py

@@ -1,6 +1,11 @@
 import hashlib
-import pickle
+import json
 
-def hash_any_object(var):
-    byte_stream = pickle.dumps(var)
-    return hashlib.sha256(byte_stream).hexdigest()
+
+def hash_any_object(var) -> str:
+    """Deterministic sha256 over a JSON-encodable value. Replaces the older
+    pickle-based hash: pickle is nondeterministic across Python versions /
+    dict-ordering tweaks and pulls more attack surface than we need for a
+    plain change-detection fingerprint."""
+    encoded = json.dumps(var, sort_keys=True, ensure_ascii=False, default=str)
+    return hashlib.sha256(encoded.encode("utf-8")).hexdigest()