fix(channels): archive group messages before the pending buffer drops them

[minhdang03]

channel_pending_messages is a buffer, not an archive. Rows leave it on two paths, both plain deletes:

• the bot being mentioned hands the whole buffer to the agent, then Clear() wipes the key;
• LLM compaction replaces old rows with a summary.

For group capture that buffer holds the only stored copy of the raw text, so a single mention or one compaction pass permanently destroyed every message nothing had read yet. Observed in production: a compaction run erased ~4.5 days of a Zalo group's history, and a later mention erased the rest along with the summary that replaced it.

Change

• New table channel_message_archive (migration 97; SQLite schema 60).
• DeleteByKey, Compact and DeleteStale copy rows into the archive inside the same transaction as the delete, tagged with archive_reason (consumed, compacted, stale).
• Archived rows keep their original id, so a replayed delete is a no-op.
• New ListArchivedByKey(channel, key, since, limit) for consumers that need full history; tenant-scoped like every other store read.

Wiring lives in the store layer, so every channel (Telegram, Discord, Slack, Feishu, WhatsApp, Zalo) and the HTTP compact endpoint are covered with no call-site changes.

Tests

• 4 SQLite unit tests (internal/store/sqlitestore/pending_message_archive_test.go)
• 4 PG integration tests (tests/integration/pending_message_archive_test.go), run against a clean pgvector instance migrating 1 → 97

Covers clear, compaction, idempotency across compact-then-clear, the since filter, and tenant scoping.

go build and go vet clean under both the default and sqliteonly tags. The 6 pre-existing Ollama failures in internal/http reproduce on a clean origin/dev checkout and are unrelated.

[clark-cant]

Approved: archive-before-delete is transactionally implemented for PostgreSQL and SQLite, tenant-scoped reads are covered, migration/schema parity is present, duplicate gate is clear, and all required CI checks pass. No critical or important findings.