feat: add post-quantum hybrid KEM + SQLCipher persistence

Feature 1 — Post-Quantum Hybrid KEM (X25519 + ML-KEM-768):
- Create hybrid_kem.rs with keygen, encrypt, decrypt + 11 unit tests
- Wire format: version(1) | x25519_eph_pk(32) | mlkem_ct(1088) | nonce(12) | ct
- Add uploadHybridKey/fetchHybridKey RPCs to node.capnp schema
- Server: hybrid key storage in FileBackedStore + RPC handlers
- Client: hybrid keypair in StoredState, auto-wrap/unwrap in send/recv/invite/join
- demo-group runs full hybrid PQ envelope round-trip

Feature 2 — SQLCipher Persistence:
- Extract Store trait from FileBackedStore API
- Create SqlStore (rusqlite + bundled-sqlcipher) with encrypted-at-rest SQLite
- Schema: key_packages, deliveries, hybrid_keys tables with indexes
- Server CLI: --store-backend=sql, --db-path, --db-key flags
- 5 unit tests for SqlStore (FIFO, round-trip, upsert, channel isolation)

Also includes: client lib.rs refactor, auth config, TOML config file support,
mdBook documentation, and various cleanups by user.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/quicnprotochat-server/Cargo.toml

@@ -32,6 +32,9 @@ quinn-proto   = { workspace = true }
 rustls        = { workspace = true }
 rcgen         = { workspace = true }
 
+# Database
+rusqlite      = { workspace = true }
+
 # Error handling
 anyhow        = { workspace = true }
 thiserror     = { workspace = true }
@@ -40,3 +43,4 @@ serde         = { workspace = true }
 
 # CLI
 clap          = { workspace = true }
+toml          = { version = "0.8" }