refactor(database): enforce Alembic-only database migrations, deprecate manual schema migrations

- Create alembic.ini and migrations/env.py for full Alembic CLI + programmatic support
- Add Alembic migration 006: detail column on processing_logs
- Add Alembic migration 007: ocr_quality_score column + drop unique filehash index
- Add Alembic migration 008: performance indexes
- Replace _run_schema_migrations() call in init_db() with _run_alembic_upgrade()
- Deprecate _run_schema_migrations() with DeprecationWarning
- Update tests for new Alembic-based approach and deprecation
- Update DatabaseConfiguration.md documentation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/versions/006_add_detail_column.py

@@ -0,0 +1,27 @@
+"""Add detail column to processing_logs table
+
+Revision ID: 006_add_detail_column
+Revises: 005_add_saved_searches
+Create Date: 2026-03-01
+
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "006_add_detail_column"
+down_revision: Union[str, None] = "005_add_saved_searches"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add detail column to processing_logs table for verbose worker log output."""
+    op.add_column("processing_logs", sa.Column("detail", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    """Remove detail column from processing_logs table."""
+    op.drop_column("processing_logs", "detail")