feat(triggers): add name column + backfill (§4.5, schema step)

Schema groundwork for the trigger `name` identity (the manifest merge key
that will let `apply` Update a trigger in place rather than only
Create/Delete). A `gen_random_uuid()` default keeps the existing write
paths valid and unique without code changes; existing rows are backfilled
to the readable `{kind}-{n}` form; `UNIQUE(app_id, name)` is enforced.

No behavior change yet — the manager-core write paths and the apply diff
start using the name in the follow-up. Verified the migration applies and
the trigger journeys (which create triggers via the unnamed path → default
name) stay green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/migrations/0046_trigger_name.sql

@@ -0,0 +1,29 @@
+-- Trigger `name` (§4.5): an explicit per-app identifier that becomes the
+-- manifest merge/upsert key (Step B switches the apply diff to key on it).
+-- Until now triggers were identified only by their per-kind semantic tuple,
+-- so the declarative tool could only Create/Delete them, never Update.
+--
+-- Backfill existing rows with `{kind}-{n}` (n = per-(app,kind) sequence by
+-- creation order) — the spec-sanctioned fallback when there's no cleaner
+-- entity token. Then enforce NOT NULL + UNIQUE(app_id, name).
+
+-- A `gen_random_uuid()` default keeps existing INSERTs (which don't yet
+-- supply a name) valid and unique — the manager-core write paths start
+-- providing meaningful names in the follow-up; new rows until then get a
+-- harmless unique placeholder.
+ALTER TABLE triggers
+    ADD COLUMN name TEXT NOT NULL DEFAULT gen_random_uuid()::text;
+
+-- Rewrite the just-defaulted existing rows to the readable `{kind}-{n}` form.
+UPDATE triggers t
+SET name = sub.nm
+FROM (
+    SELECT id,
+           kind || '-' || row_number() OVER (
+               PARTITION BY app_id, kind ORDER BY created_at, id
+           ) AS nm
+    FROM triggers
+) sub
+WHERE t.id = sub.id;
+
+CREATE UNIQUE INDEX triggers_app_name_uniq ON triggers (app_id, name);