feat(mcp-docugen): Task 1-3 config, models, auth middleware

- Settings con Pydantic Settings, validazione env obbligatori
- Shared models: TemplateVariable/Frontmatter, ImageVariable, TokenUsage, GenerationResult, TemplateSummary, TemplateAsset
- ApiKeyAuthMiddleware Bearer token con exempt paths

19 test, tutti passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/mcp-docugen/src/mcp_docugen/config.py

@@ -0,0 +1,25 @@
+from pathlib import Path
+
+from pydantic import Field, field_validator
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(
+        env_file=".env", env_file_encoding="utf-8", extra="ignore"
+    )
+
+    api_key: str = Field(..., min_length=8)
+    openrouter_api_key: str = Field(..., min_length=8)
+    openrouter_base_url: str = "https://openrouter.ai/api/v1"
+    llm_model_default: str = "anthropic/claude-sonnet-4"
+    public_base_url: str = Field(...)
+    data_dir: Path = Path("/data")
+    asset_ttl_days: int = 30
+    max_image_size_mb: int = 10
+    llm_timeout_seconds: int = 60
+
+    @field_validator("public_base_url")
+    @classmethod
+    def strip_trailing_slash(cls, v: str) -> str:
+        return v.rstrip("/")