fix(lifecycle): OAuth pw 500, invite lockout, reminder re-arm, partial-unique

Drift-audit Group 8 (lifecycle gaps):

- change_password no longer 500s for OAuth-only users: short-circuit when
  password_hash is None (verify_password would crash on None) so the route
  returns a clean 4xx instead of a 500.
- register_with_invitation no longer locks the invitee out on a username
  collision: create the user FIRST, then mark the token used, so a failed
  creation (409) leaves the single-use invite valid for retry.
- update_event re-arms reminder_sent_at when start_dt/reminder_minutes change,
  so a rescheduled event fires again instead of being permanently suppressed.
- Migration 0061: uq_topic_per_rulebook / uq_rule_per_topic become PARTIAL
  unique indexes (WHERE deleted_at IS NULL). Trashing 'X' then recreating it
  no longer 500s on the dead row's title. Model __table_args__ updated to match.

Deferred: per-occurrence reminders for recurring events (event_scheduler) —
needs a per-occurrence reminder-state design, not a one-line gate tweak.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/models/rulebook.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timezone
 
-from sqlalchemy import BigInteger, Boolean, Column, DateTime, ForeignKey, Integer, Table, Text, UniqueConstraint
+from sqlalchemy import BigInteger, Boolean, Column, DateTime, ForeignKey, Index, Integer, Table, Text, text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from fabledassistant.models import Base
@@ -42,8 +42,13 @@ class Rulebook(Base, SoftDeleteMixin):
 
 class RulebookTopic(Base, SoftDeleteMixin):
     __tablename__ = "rulebook_topics"
+    # Partial unique: a title is unique among LIVE topics in a rulebook, so a
+    # trashed topic doesn't block recreating/restoring the same title.
     __table_args__ = (
-        UniqueConstraint("rulebook_id", "title", name="uq_topic_per_rulebook"),
+        Index(
+            "uq_topic_per_rulebook", "rulebook_id", "title",
+            unique=True, postgresql_where=text("deleted_at IS NULL"),
+        ),
     )
 
     id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
@@ -76,8 +81,13 @@ class RulebookTopic(Base, SoftDeleteMixin):
 
 class Rule(Base, SoftDeleteMixin):
     __tablename__ = "rules"
+    # Partial unique: title unique among LIVE rules in a topic (soft-deleted
+    # rules don't block recreating/restoring the same title).
     __table_args__ = (
-        UniqueConstraint("topic_id", "title", name="uq_rule_per_topic"),
+        Index(
+            "uq_rule_per_topic", "topic_id", "title",
+            unique=True, postgresql_where=text("deleted_at IS NULL"),
+        ),
     )
 
     id: Mapped[int] = mapped_column(BigInteger, primary_key=True)