Assign readable ticket keys from project code and sequence.

Replace UUID-looking sync labels in TUI and web with keys like TAS-1,
while keeping the remote UUID for task-master matching.

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

internal/sqlite/migrations/005_ticket_keys.sql

@@ -0,0 +1,9 @@
+ALTER TABLE projects ADD COLUMN code TEXT NOT NULL DEFAULT '';
+ALTER TABLE projects ADD COLUMN ticket_seq INTEGER NOT NULL DEFAULT 0;
+
+ALTER TABLE tickets ADD COLUMN number INTEGER;
+ALTER TABLE tickets ADD COLUMN key TEXT NOT NULL DEFAULT '';
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_projects_code ON projects(code) WHERE code != '';
+CREATE UNIQUE INDEX IF NOT EXISTS idx_tickets_key ON tickets(key) WHERE key != '';
+CREATE UNIQUE INDEX IF NOT EXISTS idx_tickets_project_number ON tickets(project_id, number) WHERE number IS NOT NULL;