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/payloads.rs

@@ -2,6 +2,7 @@ use flate2::Compression;
 use flate2::read::GzDecoder;
 use flate2::write::GzEncoder;
 use rusqlite::Connection;
+use rusqlite::OptionalExtension;
 use sha2::{Digest, Sha256};
 use std::io::{Read, Write};
 
@@ -35,7 +36,7 @@ pub fn store_payload(conn: &Connection, options: StorePayloadOptions) -> Result<
             ),
             |row| row.get(0),
         )
-        .ok();
+        .optional()?;
 
     if let Some(id) = existing {
         return Ok(id);
@@ -74,7 +75,7 @@ pub fn read_payload(conn: &Connection, id: i64) -> Result<Option<serde_json::Val
             [id],
             |row| Ok((row.get(0)?, row.get(1)?)),
         )
-        .ok();
+        .optional()?;
 
     let Some((encoding, payload_bytes)) = row else {
         return Ok(None);