Implement SQLite index database layer

- Add pure Go SQLite driver (modernc.org/sqlite) to avoid CGO dependency
- Implement database connection management with WAL mode
- Add write mutex for serializing concurrent writes
- Create schema for all tables matching DESIGN.md specifications
- Implement repository pattern for all database entities:
  - Files, FileChunks, Chunks, Blobs, BlobChunks, ChunkFiles, Snapshots
- Add transaction support with proper rollback handling
- Add fatal error handling for database integrity issues
- Add snapshot fields for tracking file sizes and compression ratios
- Make index path configurable via VAULTIK_INDEX_PATH environment variable
- Add comprehensive test coverage for all repositories
- Add format check to Makefile to ensure code formatting

Makefile

@@ -12,9 +12,16 @@ LDFLAGS := -X 'git.eeqj.de/sneak/vaultik/internal/globals.Version=$(VERSION)' \
 all: test
 
 # Run tests
-test: lint
+test: lint fmt-check
 	go test -v ./...
 
+# Check if code is formatted
+fmt-check:
+	@if [ -n "$$(go fmt ./...)" ]; then \
+		echo "Error: Code is not formatted. Run 'make fmt' to fix."; \
+		exit 1; \
+	fi
+
 # Format code
 fmt:
 	go fmt ./...