fix: propagate DB errors instead of silently swallowing them

Replace .unwrap_or(), .ok(), and .filter_map(|r| r.ok()) patterns with
proper error propagation using ? and rusqlite::OptionalExtension where
the query may legitimately return no rows.

Affected areas:
- events_db::count_events: three count queries now propagate errors
  instead of defaulting to (0, 0) on failure
- note_parser::extract_refs_from_system_notes: row iteration errors
  are now propagated instead of silently dropped via filter_map
- note_parser::noteable_type_to_entity_type: unknown types now log a
  debug warning before defaulting to "issue"
- payloads::store_payload/read_payload: use .optional()? instead of
  .ok() to distinguish "no row" from "query failed"
- backoff::compute_next_attempt_at: use .clamp(0, 30) to guard against
  negative attempt_count, not just .min(30)
- search::vector::max_chunks_per_document: returns Result<i64> with
  proper error propagation through .optional()?.flatten()
- embedding::chunk_ids::decode_rowid: promote debug_assert to assert
  since negative rowids indicate data corruption worth failing fast on
- ingestion::dirty_tracker::record_dirty_error: use .optional()? to
  handle missing dirty_sources row gracefully instead of hard error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/events_db.rs

@@ -141,42 +141,36 @@ fn resolve_entity_ids(
 pub fn count_events(conn: &Connection) -> Result<EventCounts> {
     let mut counts = EventCounts::default();
 
-    let row: (i64, i64) = conn
-        .query_row(
-            "SELECT
+    let row: (i64, i64) = conn.query_row(
+        "SELECT
                 COUNT(CASE WHEN issue_id IS NOT NULL THEN 1 END),
                 COUNT(CASE WHEN merge_request_id IS NOT NULL THEN 1 END)
              FROM resource_state_events",
-            [],
-            |row| Ok((row.get(0)?, row.get(1)?)),
-        )
-        .unwrap_or((0, 0));
+        [],
+        |row| Ok((row.get(0)?, row.get(1)?)),
+    )?;
     counts.state_issue = row.0 as usize;
     counts.state_mr = row.1 as usize;
 
-    let row: (i64, i64) = conn
-        .query_row(
-            "SELECT
+    let row: (i64, i64) = conn.query_row(
+        "SELECT
                 COUNT(CASE WHEN issue_id IS NOT NULL THEN 1 END),
                 COUNT(CASE WHEN merge_request_id IS NOT NULL THEN 1 END)
              FROM resource_label_events",
-            [],
-            |row| Ok((row.get(0)?, row.get(1)?)),
-        )
-        .unwrap_or((0, 0));
+        [],
+        |row| Ok((row.get(0)?, row.get(1)?)),
+    )?;
     counts.label_issue = row.0 as usize;
     counts.label_mr = row.1 as usize;
 
-    let row: (i64, i64) = conn
-        .query_row(
-            "SELECT
+    let row: (i64, i64) = conn.query_row(
+        "SELECT
                 COUNT(CASE WHEN issue_id IS NOT NULL THEN 1 END),
                 COUNT(CASE WHEN merge_request_id IS NOT NULL THEN 1 END)
              FROM resource_milestone_events",
-            [],
-            |row| Ok((row.get(0)?, row.get(1)?)),
-        )
-        .unwrap_or((0, 0));
+        [],
+        |row| Ok((row.get(0)?, row.get(1)?)),
+    )?;
     counts.milestone_issue = row.0 as usize;
     counts.milestone_mr = row.1 as usize;