feat(modules): the three de-entanglement registries, with core as the registrant

Phase 2 PR 4 of docs/website/MODULE_SYSTEM.md §2.7. Adds server/src/modules/registries.js
and moves core's own notification streams, announce leg and users-detail routes
behind it, so the three seams §1.8 and §1.9 named are exercised on every boot
before any module depends on them.

Registering is validate-then-commit per registrant: the loader stages what a
module claims and the second pass commits it, so a module that throws halfway
through register() — or fails checkDeclared after it — leaves nothing behind.
That is the registry-side twin of PR 2's second-pass mount rule.

Four decisions, all the recommended option:

- announce legs became a child table. `announce_job_legs` replaces the
  towncrier_*/discord_* column groups, so the leg set is data: core registers
  `discord`, module-uo will register `towncrier`, and a module cannot ALTER a
  core table to add its own. Backfill is guarded on information_schema (a
  SELECT of a dropped column is a parse error, not a runtime one) and the
  columns go with DROP COLUMN IF EXISTS. Verified against the live dev DB:
  three legacy jobs migrated faithfully, three replays, no duplicates.
- `mapEvent` dropped from registerNotificationStreams. §1.8 already inverts the
  push path so a module owns fromShardEvent and calls core's publish() with a
  stream id it resolved; a second mapping mechanism was a leftover. The public
  safety filter, the kinds it reads and the streams it protects now live in one
  file and move together.
- core registers through the same staging area a module uses, via an explicit
  registries.registerCore() in app.js before modules.load().
- core's six /admin/users/:id/shard/* paths now go through the
  `admin.users.detail` slot, and getUser moved back to admin.controller.js.

Found on the way, and the reason two build tools changed:

- scripts/routeManifest.js could not decode a parameterised mount. Its
  unwinder expected `(?:([^\/]+?))`; express 4.22 emits `(?:\/([^/]+?))` with
  the separator inside the group. The branch had never run. It threw rather
  than guessing, which is what it is for.
- swagger-autogen cannot follow a route into an extension slot — the slot's
  router is created by declareSlot() and filled later, so there is no literal
  mount for a static parse. Regenerating deleted 407 lines and printed
  `Swagger-autogen: Success`, the spike's exact failure (MODULE_API.md §7.4).
  swagger/slotSpecs.js generates a fragment per filled slot and re-roots it at
  the prefix the router actually hangs at in the live app — read from the
  express stack via routeManifest's own mountPath, so the manifest and the spec
  cannot disagree. swagger/mergeSpec.js is the merge helper core owes for
  module fragments anyway (§6.1a), proved here against core's own slot first.

884 tests pass (856 before). routes.manifest.json is unchanged at 229 routes.
The OpenAPI spec diff is two lines of intent: the retry endpoint's summary, and
its `leg` no longer being a fixed enum.

Co-Authored-By: Claude <noreply@anthropic.com>

server/routes.guards.json

@@ -1062,7 +1062,7 @@
     {
       "method": "DELETE",
       "path": "/api/v1/admin/users/:id/shard/link/:account",
-      "handlers": 5,
+      "handlers": 4,
       "gates": [
         "noindex",
         "requireAuth",