Persist chat history to Postgres via durable JetStream consumer

DATABASE_URL required at startup; schema self-initializes.
Compose gains a postgres service for standalone dev.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.toml

@@ -21,6 +21,12 @@ tower = { version = "0.5", optional = true }
 tower-http = { version = "0.6", features = ["fs", "trace"], optional = true }
 tower-sessions = { version = "0.14", optional = true }
 async-nats = { version = "0.38", optional = true }
+sqlx = { version = "0.8", default-features = false, features = [
+    "runtime-tokio",
+    "tls-rustls",
+    "postgres",
+    "macros",
+], optional = true }
 openidconnect = { version = "4", optional = true }
 futures = { version = "0.3", optional = true }
 chrono = { version = "0.4", features = ["serde"], optional = true }
@@ -59,6 +65,7 @@ ssr = [
     "dep:tower-http",
     "dep:tower-sessions",
     "dep:async-nats",
+    "dep:sqlx",
     "dep:openidconnect",
     "dep:futures",
     "dep:chrono",