feat(worker): add github events poller

Adds the first ingestion source. Page-1 polling is ETag-conditional
(304s don't count against rate limit); the very first run paginates
back through Link "next" pages up to a 10-page safety cap so the
table starts populated rather than waiting for new activity.

Hits /users/{user}/events/public — works without auth, returns the
right scope for a public timeline. Token (GITHUB_TOKEN) is optional;
when present it raises the rate limit from 60 to 5000/hr.

New plumbing:

  moments-core::sources
    - EventSource trait (poll() -> count)
    - PollerStateStore trait (etag persistence port)
    - run_poller driver: tokio interval + jittered exponential backoff

  moments-data::github
    - GithubSource impl, raw payload preserved as JSONB
    - parse_link_next for pagination
    - 4 unit tests covering parser + Link parsing

  migration 0002_poller_state.sql
    - one row per source: source, etag, last_modified, last_fetched

Worker binary spawns one tokio task per source (just github for now)
and aborts on SIGINT. Verified by smoke-curling the upstream endpoint:
ETag and Link headers are present; payload shape matches the parser.

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

crates/moments-core/src/lib.rs

@@ -1,3 +1,7 @@
+pub mod sources;
+
+pub use sources::{EventSource, PollerState, PollerStateStore, SourceError, run_poller};
+
 use async_trait::async_trait;
 use moments_entities::{Event, EventQuery, SourceSummary};