feat: expose source_channel on activity feed

Adds p.source_channel to the SELECT and surfaces it on each event.
Migration v22 populated the column with enum values: telegram, agent,
maintenance, unknown, github. Timeline UI needs this to show per-event
provenance (2340 telegram, 698 agent, 102 maintenance, 11 unknown, 1 github).

Nulls fall back to "unknown" — only 0 rows currently null, but the
fallback is defensive for future inserts before backfill runs.

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

diagnostics/activity_feed_api.py

@@ -83,7 +83,8 @@ def _build_events():
     try:
         rows = conn.execute("""
             SELECT p.number, p.branch, p.domain, p.agent, p.submitted_by,
-                   p.merged_at, p.description, p.commit_type, p.cost_usd
+                   p.merged_at, p.description, p.commit_type, p.cost_usd,
+                   p.source_channel
             FROM prs p
             WHERE p.status = 'merged'
               AND p.commit_type = 'knowledge'
@@ -139,6 +140,7 @@ def _build_events():
                     "ci_earned": round(ci_earned, 2),
                     "summary": summary_text,
                     "pr_number": row["number"],
+                    "source_channel": row["source_channel"] or "unknown",
                 })
 
         return events, claim_activity