feat: Implement error handling and response structure for API

- Added standardized error response structure in `errors.go` for consistent error handling across the API.
- Implemented specific response functions for various HTTP status codes (400, 401, 403, 404, 500) to enhance error reporting.
- Introduced validation error handling to provide detailed feedback on input validation issues.

test: Add comprehensive tests for share handler functionality

- Created a suite of tests for share handler endpoints, covering scenarios for creating, listing, deleting shares, and managing share links.
- Included tests for permission checks, validation errors, and edge cases such as unauthorized access and invalid document IDs.

chore: Set up test utilities and database for integration tests

- Established a base handler suite for common setup tasks in tests, including database initialization and teardown.
- Implemented test data seeding to facilitate consistent testing across different scenarios.

migration: Add public sharing support in the database schema

- Modified the `documents` table to include `share_token` and `is_public` columns for managing public document sharing.
- Added constraints to ensure data integrity, preventing public documents from lacking a share token.

backend/go.mod

@@ -5,11 +5,13 @@ go 1.25.3
 require (
 	github.com/gin-contrib/cors v1.7.6
 	github.com/gin-gonic/gin v1.11.0
+	github.com/go-playground/validator/v10 v10.27.0
 	github.com/golang-jwt/jwt/v5 v5.3.0
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.3
 	github.com/joho/godotenv v1.5.1
 	github.com/lib/pq v1.10.9
+	github.com/stretchr/testify v1.11.1
 	golang.org/x/oauth2 v0.34.0
 )
 
@@ -18,20 +20,22 @@ require (
 	github.com/bytedance/sonic v1.14.0 // indirect
 	github.com/bytedance/sonic/loader v0.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.9 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
-	github.com/go-playground/validator/v10 v10.27.0 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.18.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
+	github.com/kr/text v0.2.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/quic-go/qpack v0.5.1 // indirect
 	github.com/quic-go/quic-go v0.54.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
@@ -46,4 +50,5 @@ require (
 	golang.org/x/text v0.27.0 // indirect
 	golang.org/x/tools v0.34.0 // indirect
 	google.golang.org/protobuf v1.36.9 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )