feat(search): whole-topic background message search (UI, REST, CLI, MCP)

[ritikbansod]

Implements F2 — whole-topic message search from the deep analysis: the biggest explorer gap vs Redpanda Console / AKHQ. Today's filters only apply inside a single browse window; this adds a bounded background job that scans every partition of a topic and finds the needle. Stacked on #3 (extends MutationRequests read-shaped POSTs there) — merge #1 → #2 → #3 → this.

API

• POST /api/clusters/{clusterId}/topics/{topic}/search — start: filters (valueRegex, valueContains, keyContains), optional fromTs/toTs epoch-ms bounds, limit, optional single partition → {searchId}.
• GET .../searches/{searchId} — progress (partitionsDone/Total, scanned, matched, state RUNNING|DONE|CANCELLED|FAILED) plus the matches collected so far.
• DELETE .../searches/{searchId} — cancel (consumer wakeup).

Read-only contract (same as the browser): per-job throwaway group, assign() + seeks, never commits, never triggers a rebalance. Matches reuse MessageBrowserService.toView, so schema-registry payloads come back decoded and feed the existing message detail modal + one-click reproduce flow in the UI.

Bounded by design: max 1000 results, 2M scanned messages per job, 3 concurrent jobs (400 when full), jobs expire from memory after 30 minutes. The scan stops at a snapshot end offset, so results are deterministic for the window scanned.

Role model: the search POST is classified read-shaped, so a read-only role can search; it writes nothing (and the mutation audit trail therefore does not log searches).

Surfaces

• Web UI: new Search tab in the data explorer — regex/contains/timestamp form, live progress bar, results table wired into the message detail modal.
• CLI: kview search <topic> [--value-regex RE] [--from-ts MS|ISO] [--limit N] [--wait SEC] (start + wait + print matches) and kview search-status <id>.
• MCP: kview_search_messages (start + wait, returns matches with progress; partial results carry the searchId for polling) and kview_search_status — the server now serves 18 tools.

Verification

• 4 new integration tests against an embedded broker (3 partitions): regex search finds exactly ORD-70..79 across all partitions with full scan accounting; limit stops the scan early with limitReached; fromTs in the future yields zero matches while a window covering production yields all 90; unknown searchId → 404-shaped NoSuchElementException.
• Full suite: 53/53 green; CLI --version/--help and MCP tools/list (18 tools incl. both search tools) smoke-checked.
• Not covered: cancel mid-scan is exercised only indirectly (wakeup path); UI polling verified by code review + existing patterns, not browser automation.