security: pip-audit + bandit gates that can fail; patch 29 known CVEs

security.yml previously ran safety/bandit/semgrep with `|| true` and could
not go red. Now: pip-audit on requirements.txt is a hard gate (dev deps
reported only), bandit HIGH fails (B104 bind-all skipped: intended behind
Cloudflare/Caddy), pip-licenses uploaded as a report. Weekly + on
dependency/source changes.

Running it locally found 29 advisories, all in pinned-and-forgotten
runtime deps: starlette 0.27 (7, incl. Host-header path confusion and
form DoS), fastapi 0.104, requests 2.31 (3), pymysql 1.1. Bumped to
current: fastapi 0.141.1 / starlette 1.6.0, pydantic 2.13.5, uvicorn
0.52.4, requests 2.34.2, pymysql 1.2.0; dev: pytest 9.1.1, black 26.5.1.
pip-audit is now clean. requires-python narrowed to 3.11 (the truth:
psycopg2-binary 2.9.9 fails on 3.13; pandas 2.0.3 has no 3.12 wheels).
Full suite passes; API smoke-tested (health, stations, forecast, history,
stats, docs, openapi) on the new stack. black 26 reformatted 8 files.

requirements-dev.txt

@@ -2,12 +2,12 @@
 -r requirements.txt
 
 # Testing
-pytest==7.4.3
+pytest==9.1.1
 pytest-cov==4.1.0
 pytest-asyncio==0.21.1
 
 # Code formatting and linting
-black==23.11.0
+black==26.5.1
 flake8==6.1.0
 isort==5.12.0
 mypy==1.7.1
@@ -25,5 +25,5 @@ ipython==8.17.2
 jupyter==1.0.0
 
 # Type stubs
-types-requests==2.31.0.10
+types-requests==2.33.0.20260906
 types-python-dateutil==2.8.19.14