Major refactoring: UUID-based storage, streaming architecture, and CLI improvements

This commit represents a significant architectural overhaul of vaultik:

Database Schema Changes:
- Switch files table to use UUID primary keys instead of path-based keys
- Add UUID primary keys to blobs table for immediate chunk association
- Update all foreign key relationships to use UUIDs
- Add comprehensive schema documentation in DATAMODEL.md
- Add SQLite busy timeout handling for concurrent operations

Streaming and Performance Improvements:
- Implement true streaming blob packing without intermediate storage
- Add streaming chunk processing to reduce memory usage
- Improve progress reporting with real-time metrics
- Add upload metrics tracking in new uploads table

CLI Refactoring:
- Restructure CLI to use subcommands: snapshot create/list/purge/verify
- Add store info command for S3 configuration display
- Add custom duration parser supporting days/weeks/months/years
- Remove old backup.go in favor of enhanced snapshot.go
- Add --cron flag for silent operation

Configuration Changes:
- Remove unused index_prefix configuration option
- Add support for snapshot pruning retention policies
- Improve configuration validation and error messages

Testing Improvements:
- Add comprehensive repository tests with edge cases
- Add cascade delete debugging tests
- Fix concurrent operation tests to use SQLite busy timeout
- Remove tolerance for SQLITE_BUSY errors in tests

Documentation:
- Add MIT LICENSE file
- Update README with new command structure
- Add comprehensive DATAMODEL.md explaining database schema
- Update DESIGN.md with UUID-based architecture

Other Changes:
- Add test-config.yml for testing
- Update Makefile with better test output formatting
- Fix various race conditions in concurrent operations
- Improve error handling throughout

internal/config/size.go

@@ -6,10 +6,14 @@ import (
 	"github.com/dustin/go-humanize"
 )
 
-// Size is a custom type that can unmarshal from both int64 and string
+// Size represents a byte size that can be specified in configuration files.
+// It can unmarshal from both numeric values (interpreted as bytes) and
+// human-readable strings like "10MB", "2.5GB", or "1TB".
 type Size int64
 
-// UnmarshalYAML implements yaml.Unmarshaler for Size
+// UnmarshalYAML implements yaml.Unmarshaler for Size, allowing it to be
+// parsed from YAML configuration files. It accepts both numeric values
+// (interpreted as bytes) and string values with units (e.g., "10MB").
 func (s *Size) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	// Try to unmarshal as int64 first
 	var intVal int64
@@ -34,12 +38,16 @@ func (s *Size) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	return nil
 }
 
-// Int64 returns the size as int64
+// Int64 returns the size as int64 bytes.
+// This is useful when the size needs to be passed to APIs that expect
+// a numeric byte count.
 func (s Size) Int64() int64 {
 	return int64(s)
 }
 
-// String returns the size as a human-readable string
+// String returns the size as a human-readable string.
+// For example, 1048576 bytes would be formatted as "1.0 MB".
+// This implements the fmt.Stringer interface.
 func (s Size) String() string {
 	return humanize.Bytes(uint64(s))
 }