Stream D: Security hardening — 10 fixes from QA review

- SEC-05: Fail fast if SECRET_KEY is default 'change-me-in-production'
- SEC-06: CORS allow_credentials=False (Bearer tokens don't need it)
- SEC-08: Input validation on auth schemas (min/max length, username pattern)
- SEC-10: .dockerignore files (root, backend, mcp-connector)
- SEC-11: Hardcode ALGORITHM='HS256' as constant, remove from settings
- CQ-01: Gate crawl scheduler on CRAWL_ENABLED env var (default false)
- OS-02: CONTRIBUTING.md
- OS-03: CHANGELOG.md with v1.0.0 entry
- setup.ps1: Add API_TOKEN generation (Windows parity with setup.sh)

.dockerignore

@@ -1,7 +1,11 @@
 .git
 .env
+.env.*
+!.env.example
+__pycache__
+*.pyc
 content/
-*.md
-LICENSE
-setup.sh
-setup.ps1
+node_modules/
+dist/
+*.log
+.DS_Store