fix(channels): archive group messages before the pending buffer drops them (#1514)

channel_pending_messages is a buffer, not an archive. Rows were deleted
outright on two paths: the bot being mentioned hands the buffer to the
agent and clears the key, and LLM compaction replaces old rows with a
summary. For group capture that buffer held the only copy of the raw
text, so a single mention or compaction pass destroyed days of messages
nothing had read yet.

Copy every row into channel_message_archive inside the same transaction
as the delete, tagged with the reason (consumed, compacted, stale).
Archived rows keep their original id, so a replayed delete is a no-op.
Add ListArchivedByKey so consumers that need full history read the
archive instead of the buffer.

internal/store/sqlitestore/schema.sql

@@ -1188,6 +1188,32 @@ CREATE INDEX IF NOT EXISTS idx_channel_pending_messages_lookup ON channel_pendin
 CREATE INDEX IF NOT EXISTS idx_channel_pending_messages_parent ON channel_pending_messages(channel_name, parent_history_key) WHERE parent_history_key <> '';
 CREATE INDEX IF NOT EXISTS idx_channel_pending_messages_tenant ON channel_pending_messages(tenant_id);
 
+-- ============================================================
+-- Table: channel_message_archive
+-- ============================================================
+-- Append-only copy of every pending message taken before it leaves the buffer.
+-- Rows keep their original id so a replayed delete cannot duplicate them.
+
+CREATE TABLE IF NOT EXISTS channel_message_archive (
+    id                 TEXT NOT NULL PRIMARY KEY,
+    channel_name       VARCHAR(100) NOT NULL,
+    history_key        VARCHAR(200) NOT NULL,
+    parent_history_key VARCHAR(200) NOT NULL DEFAULT '',
+    sender             VARCHAR(255) NOT NULL,
+    sender_id          VARCHAR(255) NOT NULL DEFAULT '',
+    body               TEXT NOT NULL,
+    platform_msg_id    VARCHAR(100) NOT NULL DEFAULT '',
+    is_summary         BOOLEAN NOT NULL DEFAULT 0,
+    tenant_id          TEXT NOT NULL REFERENCES tenants(id),
+    created_at         TEXT NOT NULL,
+    updated_at         TEXT NOT NULL,
+    archived_at        TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    archive_reason     VARCHAR(20) NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_channel_message_archive_lookup ON channel_message_archive(tenant_id, channel_name, history_key, created_at);
+CREATE INDEX IF NOT EXISTS idx_channel_message_archive_archived_at ON channel_message_archive(tenant_id, archived_at);
+
 -- ============================================================
 -- Table: channel_memory_extraction_runs
 -- ============================================================