feat(mcpd+db): Llm.poolName + chat dispatcher pool failover (v4 Stage 1)

Adds LB-pool-by-shared-name without introducing a new resource. The
existing `Llm.name` stays globally unique; a new optional `poolName`
column declares membership in a pool. Multiple Llms sharing a non-null
`poolName` stack into one load-balanced pool that the chat dispatcher
expands at request time.

Effective pool key = `poolName ?? name`. Solo rows (poolName=null) are
addressable as a "pool of 1" via their own name, so existing single-Llm
agents and YAMLs keep working unchanged. A solo row whose name happens
to match an explicit poolName joins the same pool — by design — so an
operator can transparently promote an existing Llm to pool seed.

Dispatcher (chat.service): prepareContext now resolves a randomly-
shuffled list of viable pool candidates (status != inactive) once per
turn. runOneInference and streamInference iterate the list on
transport-level failure (network, virtual publisher disconnect) until
one succeeds or the list is exhausted. Streaming failover only covers
"failed before first chunk" — once we've yielded text, we're committed
to that backend. Auth/4xx errors surfaced as result.status are NOT
retried; siblings with the same key/model would fail identically.

When the agent's pinned Llm is itself inactive but a sibling pool
member is up, dispatch transparently uses the sibling — that's the
whole point. When every member is inactive, prepareContext throws a
clear "No active Llm in pool '<key>' (pinned: <name>)" error rather
than letting the dispatcher's "exhausted" branch surface it.

Tests:
- 5 new chat-service tests for pool dispatch / failover / pinned-down /
  all-inactive (chat-service.test.ts).
- 7 new db schema tests for the column, the unique-name invariant, the
  fallback-to-name semantics, and the solo-name-joins-explicit-pool
  edge case (llm-pool-schema.test.ts).
- mcpd 865/865 (was 860; +5), db pool-schema 7/7, no regressions.

Stage 2 (next): HTTP route /api/v1/llms/<name>/members + aggregate pool
stats on the existing single-Llm route, CLI POOL column + describe
block + --pool-name flag, yaml round-trip.

src/db/prisma/migrations/20260427205303_add_llm_pool_name/migration.sql

@@ -0,0 +1,10 @@
+-- v4: optional pool key. When NULL, the effective pool key is the row's
+-- own `name` (pool of 1, identical to pre-v4 behavior). Multiple Llms
+-- sharing a non-null `poolName` stack into one load-balanced pool that
+-- the chat dispatcher expands at request time.
+ALTER TABLE "Llm" ADD COLUMN "poolName" TEXT;
+
+-- Index covers both the dispatcher's `WHERE poolName = $1` lookup and
+-- the v4 admin endpoint `GET /api/v1/llms/<name>/members` (which expands
+-- by effective pool key).
+CREATE INDEX "Llm_poolName_idx" ON "Llm"("poolName");