Complete foundational data layer with repository implementation

- Add GORM dependencies for SQLite and PostgreSQL
- Create domain models (User, Session, File) with common errors
- Implement repository interfaces and database layer with migrations
- Update WebApp to bootstrap with database and repositories
- Add comprehensive unit tests for repository methods
- Update config structure to support multiple database drivers
- Extend AGENTS.md with debugging principles and dependency rules

AGENTS.md

@@ -49,6 +49,18 @@ go mod tidy             # clean deps after add/remove
 
 - DO put business logic in `internal/`, keep `cmd/` thin
 - DO write all code, comments, and documentation in English
+- DO add all Go module dependencies **before** writing code that uses them
 - DON'T read `go.sum` entirely into context — use `grep` or other tools to search specific patterns if needed
 - DON'T skip `go vet ./...` before finishing work
 - DON'T commit without explicit user request
+- DON'T add, remove, or change Go module dependencies after debugging has started — ask for explicit permission first
+
+## Debugging Principles
+
+When a test failure occurs, follow this strict order:
+
+1. **Examine the test first** — ensure the test code correctly expresses the intended program behavior
+2. **Fix the test if it's wrong** — if the test doesn't represent correct expected behavior, correct the test to match the intended behavior
+3. **Fix the implementation if the test is correct** — only after confirming the test is valid, locate and fix the bug in the implementation
+4. **Never weaken tests to gain passing status** — do not relax assertions, remove edge cases, or simplify test logic just to make tests pass. Tests exist to catch problems, not to produce a 100% pass rate
+5. **Escalate after 6 rounds** — if a problem remains unresolved after 6 debugging attempts, stop and report the current state to the user for further investigation