chore: scaffold moments workspace

Cargo workspace with five crates per architecture conventions:

- moments-entities: Source enum, Event, EventQuery, SourceSummary
- moments-core:     EventReader / EventWriter ports
- moments-data:     PgStore (sqlx postgres adapter) + 0001_init.sql
- moments-api:      axum binary; /v1/{healthz,events,sources}
- moments-worker:   skeleton; pollers land in step 2

Sources committed-to for ingestion: github, gitea, hg, bugzilla.
Workstation events explicitly retired (not deferred).

Build + clippy clean. sqlx queries use the runtime API for now;
will switch to compile-time-checked macros + .sqlx offline cache
once magrathea has the moments_{ro,rw} roles and database created.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/moments-data/migrations/0001_init.sql

@@ -0,0 +1,11 @@
+CREATE TABLE events (
+    id          TEXT        PRIMARY KEY,
+    source      TEXT        NOT NULL,
+    action      TEXT        NOT NULL,
+    occurred_at TIMESTAMPTZ NOT NULL,
+    payload     JSONB       NOT NULL,
+    fetched_at  TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX events_occurred_at_desc        ON events (occurred_at DESC);
+CREATE INDEX events_source_occurred_at_desc ON events (source, occurred_at DESC);