✨ feat(bdd): magic-link BDD scenarios + bcrypt overflow fix (ADR-0028 Phase A.5)

Adds 4 BDD scenarios covering the passwordless magic-link flow:
- Happy path (request -> email arrives -> consume -> JWT)
- Token cannot be consumed twice (single-use guarantee)
- Missing token returns 400
- Unknown token returns 401

Implementation:
- features/auth/magic_link.feature with the gherkin spec
- pkg/bdd/steps/magic_link_steps.go: per-scenario unique recipient
  (`<scenario-key>-<8hex>@bdd.local`, ADR-0030), Mailpit-driven token
  extraction, regex parse of the consume URL
- pkg/bdd/steps/scenario_state.go: 2 fields added (MagicLinkEmail,
  MagicLinkToken)
- pkg/bdd/steps/steps.go: register 5 new step regexes

Bug fix exposed by the BDD run:
- pkg/user/api/magic_link_handler.go: passwordless-signup random password
  was 96 hex chars (48 bytes) which overflowed bcrypt's 72-byte input
  limit, breaking first-link signup. Reduced to 64 hex chars (32 bytes,
  256 bits entropy).

Test infra fix:
- pkg/bdd/testserver/server.go: createTestConfig() builds the
  Config literal directly (no Viper defaults), so add explicit Email +
  MagicLink config so the From address is set when the handler sends
  via local Mailpit.

Mistral wrote the feature file, magic_link_steps.go, scenario_state.go
edit, and steps.go edit autonomously in a worktree workspace. Claude
fixed the bcrypt overflow + the test-config gap exposed during verification.

Most authoring by Mistral Vibe (mistral-vibe-cli-latest).

pkg/bdd/testserver/server.go

@@ -815,6 +815,20 @@ func createTestConfig(port int, v2Enabled bool) *config.Config {
 			JWT: config.JWTConfig{
 				TTL: 24 * time.Hour,
 			},
+			// Email + MagicLink defaults so the magic-link BDD scenarios
+			// (ADR-0028 Phase A.5) can send to local Mailpit. Without these
+			// the literal Config skips Viper's SetDefault and From stays
+			// empty — pkg/email then rejects the message.
+			Email: config.EmailConfig{
+				From:     "noreply@bdd.local",
+				SMTPHost: "localhost",
+				SMTPPort: 1025,
+				Timeout:  5 * time.Second,
+			},
+			MagicLink: config.MagicLinkConfig{
+				TTL:     15 * time.Minute,
+				BaseURL: "http://localhost:8080",
+			},
 		},
 		API: config.APIConfig{
 			V2Enabled: v2Enabled,