Wire FilterBar query AST into logs/timeline queries (ticket 007)

Adds a filter evaluator in engine.rs that compiles a parsed FilterExpr
against the current index once per query (regex compiled once,
sequence/timestamp compared directly against IngestedLog fields,
other metric comparisons resolved to a HashSet<u64> of matching
sequences via a bucket scan) and evaluates it per log line with no
further index access. Wires this into EngineQuery::Logs/Timeline via
a new LogsParams::filter/TimelineParams::filter field, and adds
EngineResponse::Error so a malformed filter string surfaces as a
query-level error (mapped to 400 in api.rs).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/profile.rs

@@ -748,7 +748,13 @@ mod tests {
 
         let logs = match ask(
             &engine_tx,
-            EngineQuery::Logs(LogsParams { since_sequence: None, start_ms: None, end_ms: None, limit: None }),
+            EngineQuery::Logs(LogsParams {
+                since_sequence: None,
+                start_ms: None,
+                end_ms: None,
+                limit: None,
+                filter: None,
+            }),
         )
         .await
         {