Fix audit findings: lost updates, token signing, dependency advisories

Blocking:
- Stock adjustments read-modified-wrote outside a transaction, so concurrent
  changes silently overwrote each other. Verified: 50 concurrent -1 requests
  moved a quantity of 100 to 99 rather than 50, while all 51 history rows were
  written, leaving the ledger disagreeing with the stock. Both adjust and patch
  now take SQLite's write lock up front.
- Session tokens joined payload and HMAC with "." and split on the last
  occurrence. A raw digest can contain that byte, so ~12% of issued tokens
  failed their own validator (measured 121/1000). The digest is fixed width;
  slice by length instead.
- starlette 0.41.3 and python-multipart 0.0.20 carried 15 advisories between
  them, including a FileResponse Range-header DoS reachable through the public
  static assets. Pinned starlette explicitly; python-multipart was unused.

Also:
- PATCH quantity now writes history, and a floored adjustment logs the delta it
  applied rather than the one requested, so the log sums to the stock.
- Renaming or deleting a category or location rebuilds the search index for
  every part beneath it; full paths are indexed, so "Workshop" finds Bin A3.
- Blank names, negative quantities and explicit nulls on NOT NULL columns are
  422s instead of silent writes or 500s; taxonomy routes 404 on missing ids,
  409 on duplicates, and reject indirect parent cycles.
- Security headers, HSTS behind X-Forwarded-Proto, Secure cookie via the
  forwarded scheme, content-hashed asset URLs so Cloudflare cannot serve stale
  frontend code, and a global failed-login throttle.
- README documents a WAL-safe backup; cp of parts.db alone could lose commits.

Checks go from 68 to 134, including a suite that runs against a real server
because lost updates only appear when requests genuinely overlap.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

requirements.txt

@@ -1,4 +1,8 @@
-fastapi==0.115.6
-uvicorn[standard]==0.34.0
-pydantic==2.10.4
-python-multipart==0.0.20
+# starlette is pinned explicitly, not just inherited from fastapi's range: the
+# versions fastapi would otherwise accept include ones with published advisories
+# (notably a FileResponse Range-header CPU DoS, which matters because / and the
+# static assets are publicly reachable).
+fastapi==0.141.1
+starlette==1.6.0
+uvicorn[standard]==0.52.4
+pydantic==2.13.4