Add custom API URL and model to AI settings

- Settings → AI: optional base URL and model name fields
- Defaults to Anthropic/OpenAI public APIs when left blank
- Custom URL enables Open WebUI, LM Studio, Ollama, and any OpenAI-compatible endpoint
- Parse endpoint uses custom base URL and model if configured
- Migration 0004: ai_base_url + ai_model columns on users
- OpenAI provider label updated to "OpenAI-compatible"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/db/models/user.py

@@ -31,6 +31,8 @@ class User(Base):
 
     ai_provider: Mapped[str | None] = mapped_column(Text, nullable=True)
     ai_api_key_enc: Mapped[bytes | None] = mapped_column(LargeBinary, nullable=True)
+    ai_base_url: Mapped[str | None] = mapped_column(Text, nullable=True)
+    ai_model: Mapped[str | None] = mapped_column(Text, nullable=True)
 
     accounts: Mapped[list["Account"]] = relationship(back_populates="user", lazy="noload")  # type: ignore[name-defined]
     sessions: Mapped[list["Session"]] = relationship(back_populates="user", lazy="noload")  # type: ignore[name-defined]