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/note_parser.rs

@@ -106,8 +106,7 @@ pub fn extract_refs_from_system_notes(conn: &Connection, project_id: i64) -> Res
                 entity_id: row.get(3)?,
             })
         })?
-        .filter_map(|r| r.ok())
-        .collect();
+        .collect::<std::result::Result<Vec<_>, _>>()?;
 
     if notes.is_empty() {
         return Ok(result);
@@ -193,7 +192,10 @@ fn noteable_type_to_entity_type(noteable_type: &str) -> &str {
     match noteable_type {
         "Issue" => "issue",
         "MergeRequest" => "merge_request",
-        _ => "issue",
+        other => {
+            debug!(noteable_type = %other, "Unknown noteable_type, defaulting to issue");
+            "issue"
+        }
     }
 }