perf(db): add indexes on query-hot columns and Redis caching layer

- Add database indexes on FileRecord.created_at, FileRecord.mime_type,
  ProcessingLog.file_id, ProcessingLog.timestamp, and
  FileProcessingStep.status for faster filtering, sorting, and joins.
- Add _ensure_indexes() migration for existing databases.
- Create app/utils/cache.py with fail-open Redis GET/SET/DELETE helpers.
- Cache MIME types dropdown query in files view (120s TTL).
- Optimize batch status query to load only needed columns.

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

tests/test_database_indexes.py

@@ -0,0 +1,83 @@
+"""
+Tests for database performance indexes and schema migrations.
+"""
+
+import pytest
+from sqlalchemy import create_engine, inspect, text
+from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import StaticPool
+
+from app.database import Base, _ensure_indexes
+
+
+@pytest.fixture
+def engine_with_tables():
+    """Create an in-memory SQLite engine with all tables."""
+    engine = create_engine(
+        "sqlite:///:memory:",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
+    # Import models to register them with Base
+    from app.models import (  # noqa: F401
+        ApplicationSettings,
+        FileProcessingStep,
+        FileRecord,
+        ProcessingLog,
+    )
+
+    Base.metadata.create_all(bind=engine)
+    return engine
+
+
+@pytest.mark.unit
+def test_ensure_indexes_creates_expected_indexes(engine_with_tables):
+    """_ensure_indexes should create all performance indexes."""
+    engine = engine_with_tables
+    inspector = inspect(engine)
+
+    # Run the migration
+    _ensure_indexes(engine, inspector)
+
+    # Refresh inspector after DDL
+    inspector = inspect(engine)
+
+    expected = {
+        "files": {"ix_files_created_at", "ix_files_mime_type"},
+        "processing_logs": {"ix_processing_logs_file_id", "ix_processing_logs_timestamp"},
+        "file_processing_steps": {"ix_file_processing_steps_status"},
+    }
+
+    for table, idx_names in expected.items():
+        actual_indexes = {idx["name"] for idx in inspector.get_indexes(table)}
+        for name in idx_names:
+            assert name in actual_indexes, f"Index {name} missing from {table}; found {actual_indexes}"
+
+
+@pytest.mark.unit
+def test_ensure_indexes_idempotent(engine_with_tables):
+    """_ensure_indexes should be safe to run multiple times."""
+    engine = engine_with_tables
+    inspector = inspect(engine)
+
+    _ensure_indexes(engine, inspector)
+    # Running again should not raise
+    inspector2 = inspect(engine)
+    _ensure_indexes(engine, inspector2)
+
+
+@pytest.mark.unit
+def test_model_indexes_declared():
+    """Verify key indexes are declared in SQLAlchemy model metadata."""
+    from app.models import FileProcessingStep, FileRecord, ProcessingLog
+
+    # FileRecord.created_at should be indexed
+    assert FileRecord.__table__.c.created_at.index is True
+    # FileRecord.mime_type should be indexed
+    assert FileRecord.__table__.c.mime_type.index is True
+    # ProcessingLog.file_id should be indexed
+    assert ProcessingLog.__table__.c.file_id.index is True
+    # ProcessingLog.timestamp should be indexed
+    assert ProcessingLog.__table__.c.timestamp.index is True
+    # FileProcessingStep.status should be indexed
+    assert FileProcessingStep.__table__.c.status.index is True