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.

requirements.txt

@@ -0,0 +1,13 @@
+flask>=3.0
+
+# Security & Rate Limiting
+flask-limiter>=3.5
+flask-cors>=4.0
+
+# Observability
+prometheus-flask-exporter>=0.23
+
+# Development/Testing
+pytest>=8.0
+pytest-cov>=4.0
+locust>=2.20