fix(migrations): add _table_exists guard to migrations touching create_all()-only tables

- 2df63b7ce038: was checking _column_exists but not _table_exists.
  If llm_providers/validation_policies/llm_validation_results don't
  exist (fresh DB), _column_exists returns False and add_column crashes.
  Now checks _table_exists first.
- a1b2c3d4e5f6 (20260603_add_token_limits_to_llm_providers): no guard
  at all. llm_providers is created by create_all() at runtime. Guard
  matches pattern used by ed28d34edde7, 9f8e7d6c5b4a and others.

backend/alembic/versions/20260603_add_token_limits_to_llm_providers.py

@@ -1,5 +1,11 @@
 """Add context_window and max_output_tokens to llm_providers
 
+@RATIONALE llm_providers is created at runtime by init_db() →
+  Base.metadata.create_all(), not by any Alembic migration. On a fresh
+  database, the table doesn't exist when migrations run. Guard with
+  _table_exists() — create_all() will create the table with the model's
+  columns (which already include context_window and max_output_tokens).
+
 Revision ID: a1b2c3d4e5f6
 Revises: f1a2b3c4d5e6
 Create Date: 2026-06-03
@@ -12,6 +18,7 @@ Both NULL = use PROVIDER_DEFAULTS fallback from model name.
 from typing import Sequence, Union
 
 from alembic import op
+from sqlalchemy import inspect
 import sqlalchemy as sa
 
 
@@ -22,7 +29,15 @@ branch_labels: Union[str, Sequence[str], None] = None
 depends_on: Union[str, Sequence[str], None] = None
 
 
+def _table_exists(table_name: str) -> bool:
+    conn = op.get_bind()
+    inspector = inspect(conn)
+    return inspector.has_table(table_name)
+
+
 def upgrade() -> None:
+    if not _table_exists("llm_providers"):
+        return
     op.add_column(
         "llm_providers",
         sa.Column(
@@ -44,5 +59,7 @@ def upgrade() -> None:
 
 
 def downgrade() -> None:
+    if not _table_exists("llm_providers"):
+        return
     op.drop_column("llm_providers", "max_output_tokens")
     op.drop_column("llm_providers", "context_window")

backend/alembic/versions/2df63b7ce038_add_missing_columns_policy_id_provider_.py

@@ -18,6 +18,7 @@ from collections.abc import Sequence
 import sqlalchemy as sa
 
 from alembic import op
+from sqlalchemy import inspect
 
 # revision identifiers, used by Alembic.
 revision: str = '2df63b7ce038'
@@ -26,27 +27,35 @@ branch_labels: str | Sequence[str] | None = None
 depends_on: str | Sequence[str] | None = None
 
 
+def _table_exists(table: str) -> bool:
+    """Check if a table exists in the current database connection."""
+    conn = op.get_bind()
+    inspector = inspect(conn)
+    return inspector.has_table(table)
+
+
 def upgrade() -> None:
     """Upgrade schema."""
-    # Use IF NOT EXISTS — this is a catch-up migration for columns that
-    # may already exist (inserted by out-of-band schema changes).
     conn = op.get_bind()
 
     # Add is_multimodal to llm_providers (from 9f8e7d6c5b4a)
-    if not _column_exists(conn, "llm_providers", "is_multimodal"):
+    # llm_providers is created by create_all() at runtime — skip if not exist
+    if _table_exists("llm_providers") and not _column_exists(conn, "llm_providers", "is_multimodal"):
         op.add_column("llm_providers",
             sa.Column("is_multimodal", sa.Boolean(), nullable=False, server_default="false")
         )
         op.alter_column("llm_providers", "is_multimodal", server_default=None)
 
     # Add provider_id to validation_policies (from a7b1c2d3e4f5)
-    if not _column_exists(conn, "validation_policies", "provider_id"):
+    # validation_policies is created by create_all() at runtime — skip if not exist
+    if _table_exists("validation_policies") and not _column_exists(conn, "validation_policies", "provider_id"):
         op.add_column("validation_policies",
             sa.Column("provider_id", sa.String(), nullable=True)
         )
 
     # Add policy_id to llm_validation_results (from b1c2d3e4f5a6)
-    if not _column_exists(conn, "llm_validation_results", "policy_id"):
+    # llm_validation_results is created by create_all() at runtime — skip if not exist
+    if _table_exists("llm_validation_results") and not _column_exists(conn, "llm_validation_results", "policy_id"):
         op.add_column("llm_validation_results",
             sa.Column("policy_id", sa.String(), nullable=True, index=True)
         )