v1.1 security release: PBKDF2 passwords, session expiry, rate limiting, SSE streaming

- store.py: passwords now PBKDF2-HMAC-SHA256 (390k iters, random salt,
  constant-time compare). Legacy unsalted-SHA256 hashes upgrade transparently
  on successful login — zero impact on existing users.
- Sessions expire after 30 days (lazy cleanup on token lookup).
- Rate limiting on register (5/hr/IP), login (10/15min per IP AND per
  username), chat (30/hr/IP) — SQLite-backed, shared across gunicorn workers.
- Registration now requires 8+ char passwords.
- New /api/chat/stream SSE endpoint: citations first ('meta'), streamed
  answer deltas, 'done'. Client auto-falls back to non-streaming.
- nginx: proxy_buffering off for SSE.

astraea-nginx.conf

@@ -10,5 +10,11 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
         proxy_read_timeout 300;
         proxy_send_timeout 300;
+        # SSE streaming (/api/chat/stream) — no buffering, long read window
+        proxy_buffering off;
+        proxy_cache off;
+        proxy_http_version 1.1;
+        proxy_set_header Connection '';
+        chunked_transfer_encoding on;
     }
 }