fix(db): add message_tags schema migration (tables existed only via the optional worker)

The backend now reads message_tags for feed enrichment, packet detail and the
WebSocket initial state, but the table was created only by tagger_worker.py
startup DDL -- which exits 2 without TYPESAFE_API_KEY, so fresh stacks had no
table, WS connects were closed 1013 and the CI smoke fanout check timed out.
Idempotent create; no-op where the worker already made the tables.

backend/src/db/migrations/055_message_tags.sql

@@ -0,0 +1,51 @@
+-- 055: message_tags + tagger bookkeeping schema.
+--
+-- The core backend reads message_tags for feed enrichment, packet detail and
+-- the WebSocket initial state, but the table was previously created only by
+-- tagger_worker.py's startup DDL -- which runs after a TYPESAFE_API_KEY gate
+-- and exits before any DDL when the key is unset. Fresh stacks therefore had
+-- no table: WS clients were closed with 1013 on connect and the CI smoke's
+-- MQTT->WebSocket fanout check timed out.
+--
+-- The schema now lives in the migration ledger so every deployment has it
+-- independently of the optional worker; the worker's CREATE TABLE IF NOT
+-- EXISTS stays as a harmless no-op. Prefix 055 deliberately leaves 052-054
+-- reserved for the owner packet sharing workstream.
+
+SET LOCAL lock_timeout = '30s';
+SET LOCAL statement_timeout = '0';
+
+CREATE TABLE IF NOT EXISTS message_tags (
+  packet_hash  text PRIMARY KEY,
+  packet_time  timestamptz NOT NULL,
+  tagged_at    timestamptz NOT NULL DEFAULT now(),
+  model        text,
+  latency_ms   integer,
+  tags         jsonb NOT NULL,
+  confidence   jsonb
+);
+
+CREATE TABLE IF NOT EXISTS tagger_state (
+  id int PRIMARY KEY CHECK (id = 1),
+  cursor_time timestamptz,
+  seen bigint NOT NULL DEFAULT 0,
+  tagged bigint NOT NULL DEFAULT 0,
+  errors bigint NOT NULL DEFAULT 0,
+  http_429 bigint NOT NULL DEFAULT 0,
+  last_error text,
+  avg_latency_ms_50 real,
+  started_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz
+);
+INSERT INTO tagger_state (id) VALUES (1) ON CONFLICT DO NOTHING;
+ALTER TABLE tagger_state ADD COLUMN IF NOT EXISTS retried bigint NOT NULL DEFAULT 0;
+ALTER TABLE tagger_state ADD COLUMN IF NOT EXISTS given_up bigint NOT NULL DEFAULT 0;
+
+CREATE TABLE IF NOT EXISTS tagger_retry (
+  packet_hash      text PRIMARY KEY,
+  packet_time      timestamptz NOT NULL,
+  attempts         int NOT NULL DEFAULT 0,
+  last_error       text,
+  first_failed_at  timestamptz NOT NULL DEFAULT now(),
+  exhausted        boolean NOT NULL DEFAULT false
+);