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>

web/app.py

@@ -69,6 +69,17 @@ logger = logging.getLogger("web")
 
 apply_client = ApplyClient()
 
+# Strong refs for fire-and-forget tasks. asyncio.create_task only weakly
+# references tasks from the event loop — the GC could drop them mid-flight.
+_bg_tasks: set[asyncio.Task] = set()
+
+
+def _spawn(coro) -> asyncio.Task:
+    t = asyncio.create_task(coro)
+    _bg_tasks.add(t)
+    t.add_done_callback(_bg_tasks.discard)
+    return t
+
 
 # ---------------------------------------------------------------------------
 # App + Jinja
@@ -326,7 +337,7 @@ def _kick_apply(user_id: int, flat_id: str, url: str, triggered_by: str) -> None
         profile_snapshot=profile,
     )
 
-    asyncio.create_task(asyncio.to_thread(
+    _spawn(asyncio.to_thread(
         _finish_apply_background, app_id, user_id, flat_id, url, profile, submit_forms,
     ))