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/007_add_ocr_quality_drop_filehash_unique.py

@@ -0,0 +1,42 @@
+"""Add ocr_quality_score column and drop unique filehash index
+
+Revision ID: 007_add_ocr_quality_drop_filehash_unique
+Revises: 006_add_detail_column
+Create Date: 2026-03-01
+
+"""
+
+import logging
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+logger = logging.getLogger(__name__)
+
+# revision identifiers, used by Alembic.
+revision: str = "007_add_ocr_quality_drop_filehash_unique"
+down_revision: Union[str, None] = "006_add_detail_column"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add ocr_quality_score column and drop unique constraint on filehash."""
+    # Add ocr_quality_score column for AI-assessed text quality (0-100)
+    op.add_column("files", sa.Column("ocr_quality_score", sa.Integer(), nullable=True))
+
+    # Drop unique index on filehash to allow duplicate file records.
+    # The filehash column retains a non-unique index for lookups.
+    try:
+        op.drop_index("ix_files_filehash", table_name="files")
+    except Exception as exc:
+        logger.debug("Could not drop ix_files_filehash (may not exist): %s", exc)
+    try:
+        op.create_index("ix_files_filehash", "files", ["filehash"], unique=False)
+    except Exception as exc:
+        logger.debug("Could not create ix_files_filehash (may already exist): %s", exc)
+
+
+def downgrade() -> None:
+    """Remove ocr_quality_score column and restore unique filehash index."""
+    op.drop_column("files", "ocr_quality_score")