sessions: server-side search and incremental inventory browsing

Problem

The session API returns bounded cursor pages, but the mecatui client currently fetches every page eagerly so filtering remains entirely client-side. Large stores therefore still incur a full backend scan/list transfer before the picker becomes useful.

Desired outcome

Add real server-side session search and incremental picker browsing while preserving stable keyset pagination, owner filtering, opaque IDs, and deterministic ordering.

Scope

• server-side filters for title, exact/full ID, model, workspace, state, kind, and relationship names;
• filtering before page formation and count;
• an opaque cursor bound to the normalized query so it cannot be replayed across incompatible searches;
• mecatui debounce/cancellation and page-at-a-time loading as the operator types or scrolls;
• honest count/loading/error/empty states without retaining the entire inventory in memory.

Acceptance criteria

• Filter normalization and matching semantics are documented and identical across in-tree stores and the remote driver.
• Ownership filtering happens before search, count, and page formation; foreign rows do not affect counts/cursors/timing-visible result shape.
• Cursors are query-bound, tamper-resistant/fail-closed, deterministic under (modified_at DESC, session_id ASC), and safely best-effort under concurrent saves.
• In-tree durable stores and grpcdriver pass shared search/paging conformance.
• Mecatui cancels stale requests, debounces input, appends the next page near the viewport tail, and never replaces a newer query with an older response.
• Empty, partial-page, backend-error, unsupported-store, and concurrent-update paths are covered offline.
• Performance tests demonstrate bounded client memory/response size and no eager all-pages fetch.
• Update API docs, architecture/implementation notes, TUI docs, and user docs.

Related

• #471
• #556
• #557
• ADR 0108