feat: add parse endpoint, refactor recurring tasks, and improve web task completion

Extract CreateRecurringTask into engine package for reuse by both CLI
and API. Add POST /tasks/parse endpoint for CLI-style input parsing.
Remove FK constraint on change_log to preserve history after task
deletion. Update web frontend to filter completed tasks from view and
add mock mode support for development.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/database.go

@@ -173,13 +173,13 @@ func runMigrations() error {
 				);
 
 				-- Change log (key:value format like edit command)
+				-- No FK on task_uuid: change logs must survive task deletion
 				CREATE TABLE change_log (
 					id INTEGER PRIMARY KEY AUTOINCREMENT,
 					task_uuid TEXT NOT NULL,
 					change_type TEXT NOT NULL,
 					changed_at INTEGER NOT NULL,
-					data TEXT NOT NULL,
-					FOREIGN KEY (task_uuid) REFERENCES tasks(uuid) ON DELETE CASCADE
+					data TEXT NOT NULL
 				);
 
 				CREATE INDEX idx_change_log_timestamp ON change_log(changed_at);