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

go.mod

@@ -3,14 +3,26 @@ module git.eeqj.de/sneak/vaultik
 go 1.24.4
 
 require (
+	github.com/spf13/cobra v1.9.1
+	go.uber.org/fx v1.24.0
+	gopkg.in/yaml.v3 v3.0.1
+	modernc.org/sqlite v1.38.0
+)
+
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
-	github.com/mattn/go-sqlite3 v1.14.28 // indirect
-	github.com/spf13/cobra v1.9.1 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v0.1.9 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/spf13/pflag v1.0.6 // indirect
 	go.uber.org/dig v1.19.0 // indirect
-	go.uber.org/fx v1.24.0 // indirect
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
-	golang.org/x/sys v0.0.0-20220412211240-33da011f77ad // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
+	golang.org/x/exp v0.0.0-20250408133849-7e4ce0ab07d0 // indirect
+	golang.org/x/sys v0.33.0 // indirect
+	modernc.org/libc v1.65.10 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
 )