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/filter_parser.rs

@@ -16,9 +16,11 @@
 //! - `and` / `or` boolean combinators, with `and` binding tighter than `or`.
 //! - Parenthesized sub-expressions.
 //!
-//! Nothing in `main.rs` calls into this module yet — it is wired up by a
-//! later integration ticket — so the compiler's dead-code lint is silenced
-//! for the whole module rather than for each individual item.
+//! `engine.rs` (ticket 007) is what actually calls into this module: it
+//! compiles the returned [`FilterExpr`] against the index (resolving
+//! [`FilterValue::Raw`] tokens via `filter_datetime` and metric comparisons
+//! via bucket data) and evaluates it per log line while answering
+//! `EngineQuery::Logs`/`Timeline`.
 #![allow(dead_code)]
 
 use pest::Parser;