fix(schema): tighten tenant boundaries on resources/deps/creds/source_docs

Apply same composite-FK pattern as 001/002 for migration 003:
- resources: add UNIQUE (id, space_id) as FK target.
- resource_dependencies: denormalize space_id, composite FKs on both endpoints
  (enforces both ends of a dep live in the same space at the DB layer).
- resource_credentials: denormalize space_id, composite FK to resources.
- source_docs.resource_id: NOT NULL (tenancy anchor); resource_id was already
  absent from the update FIELDS list so docs cannot move resources.

Repos derive space_id from the resource on addDependency/addCredential so callers
can't fake cross-tenant assignment. 3 regression tests added.

tests/db/migration_003.test.js

@@ -15,4 +15,43 @@ describe('migration 003 — resources', () => {
       }
     });
   });
+
+  it('resource_dependencies rejects cross-space endpoints', async () => {
+    await withClient(async (c) => {
+      const { rows: [s1] } = await c.query(`INSERT INTO spaces(slug,name) VALUES('a','A') RETURNING id;`);
+      const { rows: [s2] } = await c.query(`INSERT INTO spaces(slug,name) VALUES('b','B') RETURNING id;`);
+      const { rows: [a] } = await c.query(
+        `INSERT INTO resources(space_id, slug, name, runtime_type) VALUES($1,'a','A','lxc') RETURNING id;`,
+        [s1.id]);
+      const { rows: [b] } = await c.query(
+        `INSERT INTO resources(space_id, slug, name, runtime_type) VALUES($1,'b','B','lxc') RETURNING id;`,
+        [s2.id]);
+      await expect(c.query(
+        `INSERT INTO resource_dependencies(resource_id, depends_on, space_id) VALUES($1,$2,$3);`,
+        [a.id, b.id, s1.id]
+      )).rejects.toThrow(/foreign key/i);
+    });
+  });
+
+  it('resource_credentials rejects assignment to a resource in another space', async () => {
+    await withClient(async (c) => {
+      const { rows: [s1] } = await c.query(`INSERT INTO spaces(slug,name) VALUES('a','A') RETURNING id;`);
+      const { rows: [s2] } = await c.query(`INSERT INTO spaces(slug,name) VALUES('b','B') RETURNING id;`);
+      const { rows: [r] } = await c.query(
+        `INSERT INTO resources(space_id, slug, name, runtime_type) VALUES($1,'r','R','lxc') RETURNING id;`,
+        [s1.id]);
+      await expect(c.query(
+        `INSERT INTO resource_credentials(resource_id, space_id, label, vault_path) VALUES($1,$2,'l','env:X');`,
+        [r.id, s2.id]
+      )).rejects.toThrow(/foreign key/i);
+    });
+  });
+
+  it('source_docs.resource_id is NOT NULL', async () => {
+    await withClient(async (c) => {
+      await expect(c.query(
+        `INSERT INTO source_docs(name, upstream_url) VALUES('n','https://x');`
+      )).rejects.toThrow(/not.null|null value/i);
+    });
+  });
 });