fix(db,topics): time.Time params for TIMESTAMP + comment-aware SQL split

Two fixes surfaced by sim e2e test (which otherwise passed full
lifecycle: created → signup_open → 3 signups → allocator → debating
→ arguments → verdict gate (409 early, 201 after debate_end_at) →
completed).

1) MySQL TIMESTAMP rejects RFC3339-with-Z strings — passing those as
   sqlx parameters fails with "Incorrect datetime value". Changed
   CreateTopicInput lifecycle fields from string to time.Time; the
   handler parses+UTCs in validateLifecycleTimes (which now returns
   the parsed array along with the validation result) and passes
   time.Time to the store. The mysql driver formats correctly.

2) splitSQL was naive `strings.Split(s, ";")` which split inside
   comments — the 001 migration had a few `--` lines containing `;`
   ("signup_close_at; immutable", etc) which broke. Migration text
   tidied to not use `;` inside comments, AND splitSQL upgraded to
   skip both `-- ...` and `/* ... */` comment regions before splitting.

Sim verified — clean apply on fresh MySQL.

internal/store/topic_store.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/google/uuid"
 	"github.com/jmoiron/sqlx"
@@ -26,10 +27,10 @@ type CreateTopicInput struct {
 	Summary         string
 	Visibility      models.Visibility
 	VerdictSchemaID string
-	SignupOpenAt    string // RFC3339; parsed by SQL
-	SignupCloseAt   string
-	DebateStartAt   string
-	DebateEndAt     string
+	SignupOpenAt    time.Time
+	SignupCloseAt   time.Time
+	DebateStartAt   time.Time
+	DebateEndAt     time.Time
 	CreatorUserID   string
 }