feat(web): list archived sessions - #391

Problem

The Web archive lifecycle tracked by #347 can hide Session entries, and #361 adds an unarchive mutation, but there is no bounded read API that can discover archived Sessions for an archive view.

Value

Archived conversations become recoverable through a stable, searchable projection without copying or rewriting canonical Pi Session data.

Approach

• Reuse Pi SessionManager metadata and the existing archived-sessions.json set as the only sources of truth.
• Add a read-only archived Session projection with a 50-item page cap, 5,000-record scan cap, and bounded normalized query.
• Use opaque cursors bound to both the last Session identity and normalized query.
• Return explicit invalid-query and stale-cursor receipts.
• Report matching rows omitted by the page and source records not scanned by the hard bound.
• Expose authenticated GET /api/sessions/archived.

This is distinct from #361 unarchive mutation work. It does not add deletion, fork, historical editing, or UI.

Validation

• bun run check
• bun run test: 1242 passed, 0 failed, 1 skipped; Vitest 30 passed
• focused adapter and Web Host tests: 33 passed
• git diff --check

Impact

• User-visible behavior: authenticated API consumers can browse archived Session metadata; no UI changes.
• Model-visible context/tools: none.
• Runtime/lifecycle: read-only bounded indexing over Pi-owned Session metadata.
• Persisted config/data: none.
• Compatibility/risk: additive endpoint and adapter method; stale cursors fail closed rather than silently skipping entries.

Related to #347.