feat: add skip segment overrides backend

internal/database/migrations/020_add_skip_segment_overrides.sql

@@ -0,0 +1,20 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS skip_segment_override (
+  id TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL,
+  anime_id INTEGER NOT NULL,
+  episode INTEGER NOT NULL,
+  skip_type TEXT NOT NULL, -- 'op' or 'ed'
+  start_time REAL NOT NULL,
+  end_time REAL NOT NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  UNIQUE(user_id, anime_id, episode, skip_type)
+);
+
+CREATE INDEX IF NOT EXISTS idx_skip_segment_override_lookup
+  ON skip_segment_override(user_id, anime_id, episode);
+
+-- +goose Down
+DROP TABLE IF EXISTS skip_segment_override;
+