Add modular app structure for L1 server refactoring

Phase 2 of the full modernization:
- App factory pattern with create_app()
- Settings via dataclass with env vars
- Dependency injection container
- Router stubs for auth, storage, api, recipes, cache, runs
- Service layer stubs for run, recipe, cache
- Repository layer placeholder

Routes are stubs that import from legacy server.py during migration.
Next: Migrate each router fully with templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/config.py

@@ -0,0 +1,72 @@
+"""
+L1 Server Configuration.
+
+Environment-based configuration with sensible defaults.
+"""
+
+import os
+from pathlib import Path
+from dataclasses import dataclass, field
+from typing import Optional
+
+
+@dataclass
+class Settings:
+    """Application settings loaded from environment."""
+
+    # Server
+    host: str = field(default_factory=lambda: os.environ.get("HOST", "0.0.0.0"))
+    port: int = field(default_factory=lambda: int(os.environ.get("PORT", "8000")))
+    debug: bool = field(default_factory=lambda: os.environ.get("DEBUG", "").lower() == "true")
+
+    # Cache
+    cache_dir: Path = field(
+        default_factory=lambda: Path(os.environ.get("CACHE_DIR", "/data/cache"))
+    )
+
+    # Redis
+    redis_url: str = field(
+        default_factory=lambda: os.environ.get("REDIS_URL", "redis://localhost:6379/5")
+    )
+
+    # Database
+    database_url: str = field(
+        default_factory=lambda: os.environ.get(
+            "DATABASE_URL", "postgresql://artdag:artdag@localhost:5432/artdag"
+        )
+    )
+
+    # IPFS
+    ipfs_api: str = field(
+        default_factory=lambda: os.environ.get("IPFS_API", "/dns/localhost/tcp/5001")
+    )
+    ipfs_gateway_url: str = field(
+        default_factory=lambda: os.environ.get("IPFS_GATEWAY_URL", "https://ipfs.io/ipfs")
+    )
+
+    # L2 Server
+    l2_server: Optional[str] = field(
+        default_factory=lambda: os.environ.get("L2_SERVER")
+    )
+    l2_domain: Optional[str] = field(
+        default_factory=lambda: os.environ.get("L2_DOMAIN")
+    )
+
+    # Derived paths
+    @property
+    def plan_cache_dir(self) -> Path:
+        return self.cache_dir / "plans"
+
+    @property
+    def analysis_cache_dir(self) -> Path:
+        return self.cache_dir / "analysis"
+
+    def ensure_dirs(self) -> None:
+        """Create required directories."""
+        self.cache_dir.mkdir(parents=True, exist_ok=True)
+        self.plan_cache_dir.mkdir(parents=True, exist_ok=True)
+        self.analysis_cache_dir.mkdir(parents=True, exist_ok=True)
+
+
+# Singleton settings instance
+settings = Settings()