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>

alembic.ini

@@ -0,0 +1,73 @@
+# Alembic Configuration File
+# Used for managing database schema migrations in DocuElevate.
+#
+# Usage:
+#   alembic upgrade head          # Apply all pending migrations
+#   alembic current               # Show current revision
+#   alembic history --verbose     # Show migration history
+#   alembic downgrade -1          # Roll back one migration
+#   alembic revision --autogenerate -m "description"  # Create new migration
+
+[alembic]
+# Path to migration scripts
+script_location = migrations
+
+# Template used to generate migration file names
+file_template = %%(rev)s_%%(slug)s
+
+# Timezone for migration file timestamps (uses UTC by default)
+# timezone =
+
+# Maximum length of characters for autogenerate revision names
+# truncate_slug_length = 40
+
+# Set to 'true' to run environment during 'revision' command
+# revision_environment = false
+
+# Set to 'true' to allow .pyc or .pyo files for migration scripts
+# sourceless = false
+
+# Version path separator; default is "os" which uses os.pathsep
+# version_path_separator = os
+
+# Output encoding for revision files
+# output_encoding = utf-8
+
+# The database URL is loaded from app.config.settings.database_url
+# in migrations/env.py, not from this file.
+sqlalchemy.url =
+
+
+[loggers]
+keys = root,sqlalchemy,alembic
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = WARN
+handlers = console
+qualname =
+
+[logger_sqlalchemy]
+level = WARN
+handlers =
+qualname = sqlalchemy.engine
+
+[logger_alembic]
+level = INFO
+handlers =
+qualname = alembic
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = NOTSET
+formatter = generic
+
+[formatter_generic]
+format = %(levelname)-5.5s [%(name)s] %(message)s
+datefmt = %H:%M:%S