Implement queue pruning and message rotation (closes #40) (#67)

Enforce `QUEUE_MAX_AGE` and `MAX_HISTORY` config values that previously existed but were not applied.

The existing cleanup loop now also:

- **Prunes `client_queues`** entries older than `QUEUE_MAX_AGE` (default 48h / 172800s)
- **Rotates `messages`** per target (channel or DM) beyond `MAX_HISTORY` (default 10000)
- **Removes orphaned messages** no longer referenced by any client queue

All pruning runs inside the existing periodic cleanup goroutine at the same interval as idle-user cleanup.

### Changes

- `internal/config/config.go`: Added `QueueMaxAge` field, reads `QUEUE_MAX_AGE` env var (default 172800)
- `internal/db/queries.go`: Added `PruneOldQueueEntries`, `PruneOrphanedMessages`, and `RotateChannelMessages` methods
- `internal/handlers/handlers.go`: Added `pruneQueuesAndMessages` called from `runCleanup`
- `README.md`: Updated data lifecycle, config table, and TODO checklist to reflect implementation

closes #40

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Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #67
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Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/db/queries.go

@@ -1109,3 +1109,45 @@ func (database *Database) GetSessionCreatedAt(
 
 	return createdAt, nil
 }
+
+// PruneOldQueueEntries deletes client output queue entries
+// older than cutoff and returns the number of rows removed.
+func (database *Database) PruneOldQueueEntries(
+	ctx context.Context,
+	cutoff time.Time,
+) (int64, error) {
+	res, err := database.conn.ExecContext(ctx,
+		"DELETE FROM client_queues WHERE created_at < ?",
+		cutoff,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"prune old client output queue entries: %w", err,
+		)
+	}
+
+	deleted, _ := res.RowsAffected()
+
+	return deleted, nil
+}
+
+// PruneOldMessages deletes messages older than cutoff and
+// returns the number of rows removed.
+func (database *Database) PruneOldMessages(
+	ctx context.Context,
+	cutoff time.Time,
+) (int64, error) {
+	res, err := database.conn.ExecContext(ctx,
+		"DELETE FROM messages WHERE created_at < ?",
+		cutoff,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"prune old messages: %w", err,
+		)
+	}
+
+	deleted, _ := res.RowsAffected()
+
+	return deleted, nil
+}