flaskpaste: initial commit with security hardening

Features:
- REST API for text/binary pastes with MIME detection
- Client certificate auth via X-SSL-Client-SHA1 header
- SQLite with WAL mode for concurrent access
- Automatic paste expiry with LRU cleanup

Security:
- HSTS, CSP, X-Frame-Options, X-Content-Type-Options
- Cache-Control: no-store for sensitive responses
- X-Request-ID tracing for log correlation
- X-Proxy-Secret validation for defense-in-depth
- Parameterized queries, input validation
- Size limits (3 MiB anon, 50 MiB auth)

Includes /health endpoint, container support, and 70 tests.

.containerignore

@@ -0,0 +1,43 @@
+# Git
+.git
+.gitignore
+
+# Python
+__pycache__
+*.py[cod]
+*$py.class
+*.so
+.Python
+venv/
+.venv/
+ENV/
+env/
+*.egg-info/
+.eggs/
+dist/
+build/
+
+# Testing
+.pytest_cache/
+.coverage
+htmlcov/
+.tox/
+tests/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# Data (use volumes instead)
+data/
+
+# Documentation (not needed in container)
+documentation/
+*.md
+!requirements.txt
+
+# Misc
+.DS_Store
+*.log