fix(server,test): W6-A stabilize CI — sync lost update, atomic recovery summary, serial PG tests

- downloads.ts observe: re-read download_task FOR UPDATE inside the tx and merge
  from the locked row; qB sync could overwrite imported back to importing when
  verifyAndImport committed between the sync read and write (E8 flake).
- agent finalize: accept summaryPrefix so the recovery verdict is written with
  the terminal status in one update (E6 read a budget_exceeded run before the
  "未恢复:…" annotation landed); recover-summary step kept, now idempotent no-op.
- organize.ts: drop local slotComplete and the once-subscription completion
  write; completion/history are decided only by applySlotEvent(verify_passed)
  (W5-C follow-up, removes double write). Manual slot organize uses the shared
  subscriptions.slotComplete.
- vitest: server split into unit (parallel) and pg (file-serial, groupOrder 1)
  projects; packages/tasks file-serial; explicit worker concurrency in e2e.
- docs: W6 stabilization record in 04-v0.1-acceptance.md.

Test expectations unchanged. Full `deno task ci` x3 green: 369s / 341s / 337s.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

deno.lock

@@ -1,6 +1,8 @@
 {
   "version": "5",
   "specifiers": {
+    "jsr:@std/internal@^1.0.14": "1.0.14",
+    "jsr:@std/path@1": "1.1.6",
     "npm:@biomejs/biome@*": "2.5.14",
     "npm:@biomejs/[email protected]": "2.5.14",
     "npm:@earendil-works/[email protected]": "[email protected]",
@@ -60,6 +62,17 @@
     "npm:[email protected]": "4.6.5",
     "npm:zod@^4.3.6": "4.6.5"
   },
+  "jsr": {
+    "@std/[email protected]": {
+      "integrity": "291516b3d4c35024d6ffbc0a9df5bf4c64116e05b50012cf846710152d2ffdf7"
+    },
+    "@std/[email protected]": {
+      "integrity": "c68485c2a4dfbb5ae3cc74fae4e8c4e5d874cf8a8ed12927917235c758b46cbe",
+      "dependencies": [
+        "jsr:@std/internal"
+      ]
+    }
+  },
   "npm": {
     "@antfu/[email protected]": {
       "integrity": "sha512-9q/yCljni37pkMr4sPrI3G4jqdIk074+iukc5aFJl7kmDCCsiJrbZ6zKxnES1Gwg+i9RcDZwvktl23puGslmvA==",