Add authentication, authorization, and team-scoped access control

Implement email/password auth with JWT sessions and API key auth for
sandbox lifecycle. Users get a default team on signup; sandboxes,
snapshots, and API keys are scoped to teams.

- Add user, team, users_teams, and team_api_keys tables (goose migrations)
- Add JWT middleware (Bearer token) for user management endpoints
- Add API key middleware (X-API-Key header, SHA-256 hashed) for sandbox ops
- Add signup/login handlers with transactional user+team creation
- Add API key CRUD endpoints (create/list/delete)
- Replace owner_id with team_id on sandboxes and templates
- Update all handlers to use team-scoped queries
- Add godotenv for .env file loading
- Update OpenAPI spec and test UI with auth flows

db/queries/teams.sql

@@ -0,0 +1,17 @@
+-- name: InsertTeam :one
+INSERT INTO teams (id, name)
+VALUES ($1, $2)
+RETURNING *;
+
+-- name: GetTeam :one
+SELECT * FROM teams WHERE id = $1;
+
+-- name: InsertTeamMember :exec
+INSERT INTO users_teams (user_id, team_id, is_default, role)
+VALUES ($1, $2, $3, $4);
+
+-- name: GetDefaultTeamForUser :one
+SELECT t.* FROM teams t
+JOIN users_teams ut ON ut.team_id = t.id
+WHERE ut.user_id = $1 AND ut.is_default = TRUE
+LIMIT 1;