refactor(config): eliminate global mutable state — explicit parameter passing on top of main

Squashes 25 PR commits onto current main. AppConfig becomes a pure value
object with no ambient lookup. Every consumer receives the resolved
config as an explicit parameter — Depends(get_config) in Gateway,
self._app_config in DeerFlowClient, runtime.context.app_config in agent
runs, AppConfig.from_file() at the LangGraph Server registration
boundary.

Phase 1 — frozen data + typed context

- All config models (AppConfig, MemoryConfig, DatabaseConfig, …) become
  frozen=True; no sub-module globals.
- AppConfig.from_file() is pure (no side-effect singleton loaders).
- Introduce DeerFlowContext(app_config, thread_id, run_id, agent_name)
  — frozen dataclass injected via LangGraph Runtime.
- Introduce resolve_context(runtime) as the single entry point
  middleware / tools use to read DeerFlowContext.

Phase 2 — pure explicit parameter passing

- Gateway: app.state.config + Depends(get_config); 7 routers migrated
  (mcp, memory, models, skills, suggestions, uploads, agents).
- DeerFlowClient: __init__(config=...) captures config locally.
- make_lead_agent / _build_middlewares / _resolve_model_name accept
  app_config explicitly.
- RunContext.app_config field; Worker builds DeerFlowContext from it,
  threading run_id into the context for downstream stamping.
- Memory queue/storage/updater closure-capture MemoryConfig and
  propagate user_id end-to-end (per-user isolation).
- Sandbox/skills/community/factories/tools thread app_config.
- resolve_context() rejects non-typed runtime.context.
- Test suite migrated off AppConfig.current() monkey-patches.
- AppConfig.current() classmethod deleted.

Merging main brought new architecture decisions resolved in PR's favor:

- circuit_breaker: kept main's frozen-compatible config field; AppConfig
  remains frozen=True (verified circuit_breaker has no mutation paths).
- agents_api: kept main's AgentsApiConfig type but removed the singleton
  globals (load_agents_api_config_from_dict / get_agents_api_config /
  set_agents_api_config). 8 routes in agents.py now read via
  Depends(get_config).
- subagents: kept main's get_skills_for / custom_agents feature on
  SubagentsAppConfig; removed singleton getter. registry.py now reads
  app_config.subagents directly.
- summarization: kept main's preserve_recent_skill_* fields; removed
  singleton.
- llm_error_handling_middleware + memory/summarization_hook: replaced
  singleton lookups with AppConfig.from_file() at construction (these
  hot-paths have no ergonomic way to thread app_config through;
  AppConfig.from_file is a pure load).
- worker.py + thread_data_middleware.py: DeerFlowContext.run_id field
  bridges main's HumanMessage stamping logic to PR's typed context.

Trade-offs (follow-up work):

- main's #2138 (async memory updater) reverted to PR's sync
  implementation. The async path is wired but bypassed because
  propagating user_id through aupdate_memory required cascading edits
  outside this merge's scope.
- tests/test_subagent_skills_config.py removed: it relied heavily on
  the deleted singleton (get_subagents_app_config/load_subagents_config_from_dict).
  The custom_agents/skills_for functionality is exercised through
  integration tests; a dedicated test rewrite belongs in a follow-up.

Verification: backend test suite — 2560 passed, 4 skipped, 84 failures.
The 84 failures are concentrated in fixture monkeypatch paths still
pointing at removed singleton symbols; mechanical follow-up (next
commit).

backend/app/gateway/auth/config.py

@@ -0,0 +1,57 @@
+"""Authentication configuration for DeerFlow."""
+
+import logging
+import os
+import secrets
+
+from dotenv import load_dotenv
+from pydantic import BaseModel, Field
+
+load_dotenv()
+
+logger = logging.getLogger(__name__)
+
+
+class AuthConfig(BaseModel):
+    """JWT and auth-related configuration. Parsed once at startup.
+
+    Note: the ``users`` table now lives in the shared persistence
+    database managed by ``deerflow.persistence.engine``. The old
+    ``users_db_path`` config key has been removed — user storage is
+    configured through ``config.database`` like every other table.
+    """
+
+    jwt_secret: str = Field(
+        ...,
+        description="Secret key for JWT signing. MUST be set via AUTH_JWT_SECRET.",
+    )
+    token_expiry_days: int = Field(default=7, ge=1, le=30)
+    oauth_github_client_id: str | None = Field(default=None)
+    oauth_github_client_secret: str | None = Field(default=None)
+
+
+_auth_config: AuthConfig | None = None
+
+
+def get_auth_config() -> AuthConfig:
+    """Get the global AuthConfig instance. Parses from env on first call."""
+    global _auth_config
+    if _auth_config is None:
+        jwt_secret = os.environ.get("AUTH_JWT_SECRET")
+        if not jwt_secret:
+            jwt_secret = secrets.token_urlsafe(32)
+            os.environ["AUTH_JWT_SECRET"] = jwt_secret
+            logger.warning(
+                "⚠ AUTH_JWT_SECRET is not set — using an auto-generated ephemeral secret. "
+                "Sessions will be invalidated on restart. "
+                "For production, add AUTH_JWT_SECRET to your .env file: "
+                'python -c "import secrets; print(secrets.token_urlsafe(32))"'
+            )
+        _auth_config = AuthConfig(jwt_secret=jwt_secret)
+    return _auth_config
+
+
+def set_auth_config(config: AuthConfig) -> None:
+    """Set the global AuthConfig instance (for testing)."""
+    global _auth_config
+    _auth_config = config