Phase 9: Event journal — append-only mutation history

- journald crate: immutable event log for every data mutation
- Events: entity_type, entity_id, field, action, old_value, new_value,
  actor, source, workspace_id, timestamp
- In-memory buffer with configurable flush threshold (default 100 events)
- Flush writes events as Parquet to journal/ directory
- Query: GET /journal/history/{entity_id} — full history of any record
- Query: GET /journal/recent?limit=50 — latest events across all entities
- Convenience methods: record_insert, record_update, record_ingest
- Stats: GET /journal/stats — buffer size, persisted file count
- Manual flush: POST /journal/flush
- Per ADR-012: events are never modified or deleted

This is the single most important future-proofing decision.
Once history is lost, it's gone forever.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/journald/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "journald"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+shared = { path = "../shared" }
+storaged = { path = "../storaged" }
+tokio = { workspace = true }
+axum = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+tracing = { workspace = true }
+arrow = { workspace = true }
+parquet = { workspace = true }
+bytes = { workspace = true }
+chrono = { workspace = true }
+object_store = { workspace = true }