feat(server): connection pool, session persistence, blob storage in SqlStore

- Replace Mutex<Connection> with Vec<Mutex<Connection>> pool (default 4)
  with try_lock fast-path and blocking fallback
- Add SessionRecord struct and session CRUD to Store trait (default no-ops)
- Implement session persistence in SqlStore (sessions table, migration 009)
- Add blob upload/download with SHA-256 verified staging assembly
  (blobs + blob_staging tables, migration 010)
- All 35 server tests pass, FileBackedStore unaffected

crates/quicproquo-server/migrations/009_sessions.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS sessions (
+    token BLOB PRIMARY KEY,
+    username TEXT NOT NULL,
+    identity_key BLOB NOT NULL,
+    created_at INTEGER NOT NULL,
+    expires_at INTEGER NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_sessions_expires ON sessions(expires_at);