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/ingestion/dirty_tracker.rs

@@ -1,4 +1,5 @@
 use rusqlite::Connection;
+use rusqlite::OptionalExtension;
 
 use crate::core::backoff::compute_next_attempt_at;
 use crate::core::error::Result;
@@ -88,11 +89,17 @@ pub fn record_dirty_error(
     error: &str,
 ) -> Result<()> {
     let now = now_ms();
-    let attempt_count: i64 = conn.query_row(
-        "SELECT attempt_count FROM dirty_sources WHERE source_type = ?1 AND source_id = ?2",
-        rusqlite::params![source_type.as_str(), source_id],
-        |row| row.get(0),
-    )?;
+    let attempt_count: Option<i64> = conn
+        .query_row(
+            "SELECT attempt_count FROM dirty_sources WHERE source_type = ?1 AND source_id = ?2",
+            rusqlite::params![source_type.as_str(), source_id],
+            |row| row.get(0),
+        )
+        .optional()?;
+
+    let Some(attempt_count) = attempt_count else {
+        return Ok(());
+    };
 
     let new_attempt = attempt_count + 1;
     let next_at = compute_next_attempt_at(now, new_attempt);