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/metric_primitive.rs

@@ -67,7 +67,11 @@ pub enum Number {
 }
 
 impl Number {
-    fn as_f64(&self) -> f64 {
+    /// Widens to `f64` for cross-kind numeric comparisons (e.g. a filter
+    /// query's `FilterValue::Number` against a declared metric's captured
+    /// `Number`). `pub(crate)` rather than private since ticket 007's filter
+    /// evaluator (in `engine.rs`) needs it and lives in a different module.
+    pub(crate) fn as_f64(&self) -> f64 {
         match self {
             Number::Int(v) => *v as f64,
             Number::Float(v) => *v,