fix: tool start hanging + SSE 429 errors

1. Remove Docker build from create_instance for manifest types — the build
   was blocking the HTTP request for several minutes, causing frontend
   timeouts and retries. Image is now built lazily on start (via the
   existing _prepare_manifest_instance path in start_instance).

2. Increase MAX_CONNECTIONS_PER_USER from 5 to 20 for SSE endpoint —
   aggressive reconnect loops from the frontend were exhausting the limit
   and causing 429 errors unrelated to tool starting.

Quality gates: ruff clean, tsc --noEmit clean, pytest workspaces (9 passed)

apps/api/src/services/terminal_manager.py

@@ -91,14 +91,18 @@ class TerminalManager:
         """
         try:
             async with SessionLocal() as db_session:
-                stmt = pg_insert(TerminalSessionModel).values(
-                    id=uuid.UUID(session_id),
-                    instance_id=instance_id,
-                    name=name,
-                    status="active",
-                    created_at=datetime.now(timezone.utc),
-                    last_activity_at=datetime.now(timezone.utc),
-                ).on_conflict_do_nothing(index_elements=["id"])
+                stmt = (
+                    pg_insert(TerminalSessionModel)
+                    .values(
+                        id=uuid.UUID(session_id),
+                        instance_id=instance_id,
+                        name=name,
+                        status="active",
+                        created_at=datetime.now(timezone.utc),
+                        last_activity_at=datetime.now(timezone.utc),
+                    )
+                    .on_conflict_do_nothing(index_elements=["id"])
+                )
                 await db_session.execute(stmt)
                 await db_session.commit()
                 logger.debug(