fix(#365): fix ruff, mypy, pip, and bandit issues in coordinator

- Fix 20 ruff errors: UP035 (Callable import), UP042 (StrEnum), E501
  (line length), F401 (unused imports), UP045 (Optional -> X | None),
  I001 (import sorting)
- Fix mypy error: wrap slowapi rate limit handler with
  Exception-compatible signature for add_exception_handler
- Pin pip >= 25.3 in Dockerfile (CVE-2025-8869, CVE-2026-1703)
- Add nosec B104 to config.py (container-bound 0.0.0.0 is acceptable)
- Add nosec B101 to telemetry.py (assert for type narrowing)
- Create bandit.yaml to suppress B404/B607/B603 in gates/ tooling

Fixes #365

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/coordinator/src/telemetry.py

@@ -139,7 +139,7 @@ class TelemetryService:
         if self._tracer is None:
             # Initialize if not already done
             self.initialize()
-        assert self._tracer is not None
+        assert self._tracer is not None  # nosec B101 — Type narrowing after None guard
         return self._tracer
 
     def shutdown(self) -> None: