feat: add persistent on-disk HTTP response cache

[mkarim1378]

Adds a two-tier caching system: the existing in-memory ResponseCache (L1, fast, lost on restart) is now backed by a new on-disk DiskCache (L2, persistent, survives restarts).

How it works:

• Cache lookup: check memory first; on miss check disk; on disk hit promote the entry back into memory for future requests.
• Cache write: write to both memory and disk simultaneously.
• Each disk entry is a single binary file: 8-byte expiry timestamp (little-endian double) followed by the raw HTTP response bytes. Atomic rename guarantees no partial writes.
• Eviction: expired files removed on every put(); oldest-first deletion enforces the max_mb size cap.
• Existing TTL and cacheability logic (Cache-Control, no-store, Set-Cookie, static-asset heuristics) is reused unchanged.

New files:

• src/core/disk_cache.py: DiskCache class (get/put/evict/stats)

Changed files:

• src/proxy/proxy_server.py: initialise DiskCache from config; L2 lookup and write at both HTTPS-MITM and plain-HTTP paths
• config.example.json: disk_cache section (enabled, max_mb, dir)
• .gitignore: exclude http_cache/ directory

Config (off by default):
"disk_cache": { "enabled": true, "max_mb": 200, "dir": "" }