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/alembic/versions/0004_ai_base_url.py

@@ -0,0 +1,23 @@
+"""add ai_base_url and ai_model to users
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-04-22
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0004"
+down_revision = "0003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("ai_base_url", sa.Text, nullable=True))
+    op.add_column("users", sa.Column("ai_model", sa.Text, nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("users", "ai_model")
+    op.drop_column("users", "ai_base_url")