feat: add pg_replication module for reactive WAL-based cache invalidation

Implements a PostgreSQL logical replication module that streams WAL
changes, buffers them per XID for transaction atomicity, and publishes
CacheKey invalidation events to the existing framework broadcast channel.

Key design choices:
- Transaction atomicity: changes are buffered until COMMIT so the fanout
  always receives a consistent view. A 1000-row bulk-insert emits one
  table-level CacheKey, not 1000 row events.
- Native protocol: uses a self-contained raw TCP + postgres-protocol
  implementation for the replication connection (tokio-postgres 0.7 does
  not expose copy_both_simple publicly).
- Corrected pgoutput v1 parser: original maybe_sql_integration code
  incorrectly read XID from DML messages; DML messages carry no XID in
  proto v1 — only Begin does.
- Clean integration: publishes CacheKey::Channel events to store.events
  so the existing fanout task picks them up with zero changes to the
  fanout loop.
- New cx.subscribe(key) API on RenderContext for manually registering
  subscription keys (needed when queries go through PgPool, not cx.run).

New files:
  src/pg_replication/mod.rs       — PgReplicationListener, key helpers
  src/pg_replication/wal_parser.rs — pgoutput v1 binary protocol parser
  src/pg_replication/emitter.rs   — WalEmitter (tx-buffering + CacheKey emit)
  src/pg_replication/proto.rs     — raw TCP PG wire-protocol connection
  examples/pg_replication/main.rs — live message board example
  docker-compose.yml              — PG 16 container with wal_level=logical

Run the example:
  docker compose up -d
  cargo run --example pg_replication --features pg_replication

https://claude.ai/code/session_01SLGgXeqKV2o7KTmCaQZZPg

Cargo.toml

@@ -3,14 +3,20 @@ name = "livevue-rs"
 version = "0.1.0"
 edition = "2021"
 
-# Run the todo example with: cargo run --example todo
+# Run the todo example with:          cargo run --example todo
+# Run the pg_replication example with: cargo run --example pg_replication --features pg_replication
+
+[features]
+# Opt-in feature: PostgreSQL logical replication for reactive cache invalidation.
+# Adds tokio-postgres and bytes as dependencies.
+pg_replication = ["dep:tokio-postgres", "dep:postgres-protocol", "dep:bytes"]
 
 [dependencies]
 axum = { version = "0.7", features = ["macros"] }
 tokio = { version = "1", features = ["full"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite"] }
+sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite", "postgres"] }
 maud = { version = "0.26", features = ["axum"] }
 uuid = { version = "1", features = ["v4", "serde"] }
 dashmap = "6"
@@ -19,6 +25,16 @@ tokio-stream = "0.1"
 tracing = "0.1.44"
 futures = "0.3.32"
 
+# Optional: PostgreSQL logical replication
+tokio-postgres = { version = "0.7", optional = true }
+postgres-protocol = { version = "0.6", optional = true }
+bytes = { version = "1", optional = true }
 
 [dev-dependencies]
-chrono = { version = "0.4", features = ["clock"] }
+chrono = { version = "0.4", features = ["clock"] }
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+
+[[example]]
+name = "pg_replication"
+path = "examples/pg_replication/main.rs"
+required-features = ["pg_replication"]