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/db/db.go

@@ -105,10 +105,33 @@ func RunMigrations(ctx context.Context, d *sqlx.DB) error {
 }
 
 func splitSQL(s string) []string {
-	// Crude but adequate for our migrations (no string-literal semicolons).
-	// If we ever need to embed semicolons inside strings, switch to a
-	// proper SQL tokenizer.
-	return strings.Split(s, ";")
+	// Comment-aware splitter: skip `;` inside `-- ...` line comments
+	// and `/* ... */` block comments. Doesn't handle string-literal
+	// semicolons (we don't put any) — if we ever need that, swap in a
+	// real SQL tokenizer.
+	var b strings.Builder
+	i := 0
+	for i < len(s) {
+		if i+1 < len(s) && s[i] == '-' && s[i+1] == '-' {
+			// single-line comment — strip through end of line
+			for i < len(s) && s[i] != '\n' {
+				i++
+			}
+			continue
+		}
+		if i+1 < len(s) && s[i] == '/' && s[i+1] == '*' {
+			// block comment — strip through `*/`
+			i += 2
+			for i+1 < len(s) && !(s[i] == '*' && s[i+1] == '/') {
+				i++
+			}
+			i += 2
+			continue
+		}
+		b.WriteByte(s[i])
+		i++
+	}
+	return strings.Split(b.String(), ";")
 }
 
 func firstLine(s string) string {