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};
 

crates/moments-core/src/sources.rs

@@ -0,0 +1,96 @@
+use std::{sync::Arc, time::Duration};
+
+use async_trait::async_trait;
+use chrono::{DateTime, Utc};
+use tracing::{debug, info, warn};
+
+use crate::StoreError;
+
+/// A pollable upstream activity feed (github, gitea, hg, bugzilla).
+///
+/// Implementations are responsible for: fetching from upstream, persisting
+/// any incremental-fetch bookkeeping (etag, since-cursor), transforming
+/// raw payloads into [`moments_entities::Event`] rows, and writing them.
+/// `poll` returns the count of rows ingested on this tick (0 if nothing
+/// changed upstream).
+#[async_trait]
+pub trait EventSource: Send + Sync {
+    fn name(&self) -> &'static str;
+    async fn poll(&self) -> Result<usize, SourceError>;
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum SourceError {
+    #[error("http: {0}")]
+    Http(String),
+    #[error("parse: {0}")]
+    Parse(String),
+    #[error("storage: {0}")]
+    Storage(#[from] StoreError),
+}
+
+/// Persisted per-source bookkeeping for incremental fetch (etag, last-modified).
+#[derive(Debug, Clone)]
+pub struct PollerState {
+    pub source: String,
+    pub etag: Option<String>,
+    pub last_modified: Option<DateTime<Utc>>,
+    pub last_fetched: DateTime<Utc>,
+}
+
+#[async_trait]
+pub trait PollerStateStore: Send + Sync {
+    async fn load(&self, source: &str) -> Result<Option<PollerState>, StoreError>;
+    async fn save(
+        &self,
+        source: &str,
+        etag: Option<&str>,
+        last_modified: Option<DateTime<Utc>>,
+    ) -> Result<(), StoreError>;
+    async fn touch(&self, source: &str) -> Result<(), StoreError>;
+}
+
+/// Drive a single source on a fixed interval until cancelled. Backs off
+/// (with jitter) on consecutive failures up to a 64-second ceiling.
+pub async fn run_poller(source: Arc<dyn EventSource>, interval: Duration) {
+    let mut ticker = tokio::time::interval(interval);
+    ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+
+    let mut consecutive_failures: u32 = 0;
+    loop {
+        ticker.tick().await;
+        match source.poll().await {
+            Ok(0) => {
+                consecutive_failures = 0;
+                debug!(source = source.name(), "no new events");
+            }
+            Ok(count) => {
+                consecutive_failures = 0;
+                info!(source = source.name(), count, "ingested");
+            }
+            Err(e) => {
+                consecutive_failures = consecutive_failures.saturating_add(1);
+                let backoff = backoff_with_jitter(consecutive_failures);
+                warn!(
+                    source = source.name(),
+                    error = %e,
+                    attempt = consecutive_failures,
+                    backoff_ms = backoff.as_millis() as u64,
+                    "poll failed; backing off"
+                );
+                tokio::time::sleep(backoff).await;
+            }
+        }
+    }
+}
+
+fn backoff_with_jitter(attempt: u32) -> Duration {
+    // 1s, 2s, 4s, 8s, 16s, 32s, 64s ... capped
+    let base_ms: u64 = 1_000u64.saturating_mul(1u64 << attempt.min(6));
+    // pseudo-random jitter from system time nanos — fine for backoff smoothing.
+    let jitter_ms = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .map(|d| (d.subsec_nanos() % 1_000) as u64)
+        .unwrap_or(0);
+    Duration::from_millis(base_ms + jitter_ms)
+}