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>

apply/main.py

@@ -1,3 +1,4 @@
+import hmac
 import logging
 from contextlib import asynccontextmanager
 from urllib.parse import urljoin, urlparse
@@ -71,7 +72,7 @@ class ApplyResponse(BaseModel):
 def require_api_key(x_internal_api_key: str | None = Header(default=None)) -> None:
     if not INTERNAL_API_KEY:
         raise HTTPException(status_code=503, detail="INTERNAL_API_KEY not configured")
-    if x_internal_api_key != INTERNAL_API_KEY:
+    if not x_internal_api_key or not hmac.compare_digest(x_internal_api_key, INTERNAL_API_KEY):
         raise HTTPException(status_code=401, detail="invalid api key")