fix(memory-plugin): emit JSON literal "null" for nil metadata/propagation

URGENT — fixes a production regression introduced by #1794 hitting a
latent bug in the plugin's marshalMetadata helper.

Symptom: every POST /workspaces/:id/memories on tenants running the
post-#1794 image returns HTTP 500. Tenant logs show:

    Commit memory error (plugin): memory-plugin: internal:
    commit memory: pq: invalid input syntax for type json

Same error fires for UpsertNamespace called from the Phase A2 backfill
(cmd/memory-backfill), causing the client's circuit breaker to open
after 3 failures and rejecting EVERY subsequent plugin call for that
process.

Root cause: marshalMetadata returned Go nil for nil-map input. lib/pq
sends Go nil as a bytea-typed NULL placeholder. PostgreSQL refused to
implicitly cast that placeholder into the target jsonb column
(memory_namespaces.metadata + memory_records.propagation), so the
INSERT failed at parse time. Direct `INSERT ... metadata=NULL` works
fine from psql, so the bug is purely on the driver-binding side —
sending the JSON literal `null` keeps the parameter typed as a
valid jsonb value regardless of pq's auto-conversion heuristics.

Existing sqlmock tests use sqlmock.AnyArg() for the metadata column,
so they didn't catch the real-postgres type mismatch.

Verified:
- All four production tenants on staging-39c8618 currently FAIL every
  POST /memories with the JSON syntax error
- Direct `INSERT INTO memory_plugin.memory_namespaces ... NULL` from
  psql succeeds — confirms PostgreSQL is fine with NULL; only the
  pq-binding path is broken
- pgplugin unit test updated to pin the new contract: nil input →
  `[]byte("null")`

Once this lands + image rebuilds + tenants recycle:
- POST /memories writes will succeed (closes the #1794 regression)
- Phase A2 backfill via /memory-backfill will succeed (closes #1791
  step 2)

Closes the production regression. Phase A2 backfill is now unblocked.

workspace-server/internal/memory/pgplugin/store.go

@@ -349,8 +349,23 @@ func scanMemory(row interface {
 }
 
 func marshalMetadata(m map[string]interface{}) ([]byte, error) {
+	// Returning Go nil here previously made lib/pq send the parameter
+	// as a bytea-typed NULL placeholder that PostgreSQL refused to
+	// implicitly cast into the jsonb column. The resulting INSERT
+	// failed with `pq: invalid input syntax for type json`, which
+	// surfaced first as "scan namespace" errors from the QueryRow
+	// path (the row never gets created so RETURNING produces no rows;
+	// the QueryRow error propagates through Scan). Caught 2026-05-24
+	// during the Phase A2 backfill — UpsertNamespace failed on every
+	// call, then the client's circuit breaker opened and ALL plugin
+	// writes started failing. The same hazard exists on the
+	// memory_records.propagation column (also marshalled via this
+	// helper). Sending the JSON literal `null` keeps the parameter
+	// typed as a valid jsonb value regardless of pq's auto-conversion
+	// heuristics — semantically identical to NULL for our consumers
+	// (they treat empty propagation/metadata as absent).
 	if m == nil {
-		return nil, nil
+		return []byte(`null`), nil
 	}
 	b, err := json.Marshal(m)
 	if err != nil {

workspace-server/internal/memory/pgplugin/store_test.go

@@ -17,12 +17,19 @@ import (
 // --- marshalMetadata corner cases ---
 
 func TestMarshalMetadata_Nil(t *testing.T) {
+	// Post-fix (2026-05-24): nil input must return the JSON literal
+	// `null` (4 bytes), NOT Go nil. Returning Go nil made lib/pq send
+	// a bytea NULL parameter that PostgreSQL refused to implicitly
+	// cast into the target jsonb column, breaking every UpsertNamespace
+	// and CommitMemory call with `pq: invalid input syntax for type
+	// json`. The Phase A2 backfill exposed it (production saw every
+	// POST /memories return 500 immediately post-tenant-recycle).
 	got, err := marshalMetadata(nil)
 	if err != nil {
 		t.Errorf("err = %v", err)
 	}
-	if got != nil {
-		t.Errorf("got = %v, want nil", got)
+	if string(got) != "null" {
+		t.Errorf("got = %q (%v), want %q", string(got), got, "null")
 	}
 }