Add ticket file attachments with simple in-browser Excel editing.

Store uploads on disk under DataDir and expose them in the web ticket view so .xlsx files can be previewed and saved as cell values without YouTrack or TUI sync.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/sqlite/migrations/004_attachments.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS ticket_attachments (
+  id           TEXT PRIMARY KEY,
+  ticket_id    TEXT NOT NULL REFERENCES tickets(id) ON DELETE CASCADE,
+  project_id   TEXT NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+  filename     TEXT NOT NULL,
+  content_type TEXT NOT NULL DEFAULT 'application/octet-stream',
+  size_bytes   INTEGER NOT NULL,
+  created_by   TEXT NOT NULL REFERENCES users(id),
+  created_at   TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_attachments_ticket ON ticket_attachments(ticket_id);