feat(fleet): native Mosaic backlog on @mosaicstack/db (atomic claim + TTL)

Add Mosaic's own backlog-of-record on the existing Postgres storage layer,
replacing the former Hermes adapter (no Hermes dependency) and the earlier
sqlite idea (no sqlite, no new client).

- packages/db: `backlog` table (drizzle) + enum + migration 0011; BacklogService
  taking a Db handle so it runs on both createDb (server PG) and createPgliteDb
  (embedded). Atomic claim via SELECT ... FOR UPDATE SKIP LOCKED inside one tx,
  TTL claims + reclaim of expired, deps DAG gate, idempotency-key dedup, stats.
- packages/mosaic: `mosaic fleet backlog <sub> --json` (create/list/claim/
  reclaim/link/stats/block/complete). Embedded PGlite default at
  <mosaicHome>/fleet/backlog; full Postgres via DATABASE_URL — same code.
  Migrations run on first use.
- install.sh: preserve fleet/backlog across `mosaic update`.
- docs/fleet/backlog-conventions.md: schema, phase convention, atomic-claim +
  TTL semantics, PGlite-default/Postgres-by-config, Mosaic-native (no Hermes).
- Tests (in-memory PGlite, real PG semantics): create/list, exactly-one-winner
  concurrency (and N-card no-double-claim), deps gate, reclaim of expired, stats.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/db/drizzle/0011_bitter_gateway.sql

@@ -0,0 +1,22 @@
+CREATE TYPE "public"."backlog_status" AS ENUM('ready', 'claimed', 'blocked', 'done');--> statement-breakpoint
+CREATE TABLE "backlog" (
+	"id" text PRIMARY KEY NOT NULL,
+	"title" text NOT NULL,
+	"body" text,
+	"phase" text,
+	"priority" integer DEFAULT 0 NOT NULL,
+	"status" "backlog_status" DEFAULT 'ready' NOT NULL,
+	"depends_on" jsonb DEFAULT '[]'::jsonb NOT NULL,
+	"claim_owner" text,
+	"claim_ttl_seconds" integer,
+	"claimed_at" timestamp with time zone,
+	"attempts" integer DEFAULT 0 NOT NULL,
+	"idempotency_key" text,
+	"acceptance" jsonb,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX "backlog_status_priority_idx" ON "backlog" USING btree ("status","priority");--> statement-breakpoint
+CREATE INDEX "backlog_status_claimed_at_idx" ON "backlog" USING btree ("status","claimed_at");--> statement-breakpoint
+CREATE UNIQUE INDEX "backlog_idempotency_key_idx" ON "backlog" USING btree ("idempotency_key");