fix: resolve file deletion conflict, empty paths, timestamp comparison and event mutation (#149)

[ranadheer-designs]

Fixes #149
/claim #149

Summary

Resolves four file and event consistency bugs in file-server:

1. Conflicting delete selectors (routes/files/delete.ts):
   • Supplying both file_id and file_path now returns HTTP 400 ("Provide only one of file_id or file_path"), preventing ctx.db.deleteFile from deleting two files in a single request.
2. Empty file path validation (routes/files/upsert.ts & routes/files/rename.ts):
   • Validates normalizePath(file_path) and normalizePath(new_file_path). If it resolves to empty (such as "" or "/"), returns HTTP 400 with descriptive error ("file_path must name a file" / "new_file_path must name a file").
3. Timestamp instant comparison (lib/db/db-client.ts):
   • listEvents now compares since using Date.parse() epoch numbers rather than lexicographical string comparison, so timestamps with timezone offsets (e.g. +02:00, -01:00) evaluate accurately.
4. Event snapshot immutability (lib/db/db-client.ts):
   • renameFile now returns a newly allocated events array [...state.events, createdEvent, deletedEvent] instead of mutating state.events in-place, preserving earlier snapshots.

Tests

Added 4 dedicated regression test suites:

• tests/routes/selector-conflict.test.ts: verifies conflicting delete selectors return 400 and preserve both files.
• tests/routes/empty-file-path.test.ts: verifies empty or root file paths on upsert and rename are rejected with 400.
• tests/routes/event-timestamp.test.ts: verifies since correctly filters events with timezone offsets.
• tests/rename-events.test.ts: verifies renameFile preserves previous event array snapshots.

All 33 tests pass locally with bun test and typecheck passes with bunx tsc --noEmit.

[ranadheer-designs]

cc @seveibar @looternikhil-cpu (issue author)

All CI checks are passing (3/3 green). Fixes #149 covering the 4 consistency fixes (delete selector conflict handling, empty path validation, timestamp parsing with timezones, and event snapshot immutability) with dedicated regression tests.

Ready for review!