Add storage retention, fix reverse proxy port and replay freeze

[adn8naiagent]

Storage management (requested by @db-can):

• storage.delete/delete_prefix for local and R2 (batched at 1000 keys)
• delete_session removes replay.json first, so an interrupted delete reads as "not downloaded" and reprocesses cleanly instead of half-removed
• retention.py: policy stored in config/retention.json, applied on save and swept daily by its own task. Not in auto_precompute_loop, which only runs Fri-Mon and isn't started at all when AUTO_PRECOMPUTE=off
• a delete tombstone stops auto-precompute re-downloading a removed session
• evict_cached_session frees replay frames from memory on delete
• Storage panel in the picker header; per-session delete on right-click

Fixes:

• docker-compose: container always listens on 8000 so reverse proxies have a stable target; PORT now sets the published host port only. 2.1.2 passed PORT into the container, moving the port Traefik/nginx/cloudflared target
• replay websocket sends a keep-alive every 20s, and the client reconnects with backoff and resumes in place, so a dropped socket no longer freezes the UI permanently ([Bug] UI freezes at ~4:45 behind reverse proxies (Regression in v2.1.2) #106)
• replace the 50ms receive_text cancellation with a queue-fed reader task
• clear the events cache after processing so downloaded markers appear immediately rather than after the 5 minute TTL