remove(llm): retire think_enabled setting entirely

Two-in-one cleanup motivated by the chat hang in dev 2026-05-22.

The crash root cause from the guarded-task traceback:

    UnboundLocalError: cannot access local variable 'get_setting'
    where it is not associated with a value
      File generation_task.py:257, in run_generation
        think = (await get_setting(user_id, 'think_enabled', 'false'))...

generation_task.py imports get_setting at module top, but a later
'if voice_mode: from ... import get_setting' block scopes it as a
function-local. When voice_mode=False the local import never runs,
but Python had already flagged get_setting as local for the entire
body — the think_enabled read at line 257 hit UnboundLocalError.

The line itself was dead-weight anyway. With the conversation+curator
architecture: chat ships tools=[] (think on a no-tools pass is pure
latency cost; nothing for the model to reason ABOUT in tool-call
terms), and the curator hardcodes think=False already. The user
setting was a holdover from before the architecture pivot. Removing
it entirely is cleaner than fixing the scoping bug to preserve a
toggle nobody should be using:

- generation_task.py: think hardcoded False. Removed the get_setting
  call (which fixes the UnboundLocalError as a side effect).
- SettingsView.vue: dropped the Enable model thinking checkbox, the
  thinkEnabled / savingThinkEnabled refs, the saveThinkEnabled
  function, and the think_enabled load step.
- Migration 0050: DELETE FROM settings WHERE key='think_enabled'
  to clean up any stored rows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

alembic/versions/0050_drop_think_enabled_setting.py

@@ -0,0 +1,27 @@
+"""drop stored think_enabled rows — setting was removed
+
+Revision ID: 0050
+Revises: 0049
+Create Date: 2026-05-23
+
+The `think_enabled` user setting was retired with the chat+curator
+architecture: chat has tools=[] and curator hardcodes think=False, so
+the toggle was dead weight. Any rows already in `settings` for that
+key are now unread by the app; this migration clears them.
+"""
+from alembic import op
+
+
+revision = "0050"
+down_revision = "0049"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("DELETE FROM settings WHERE key = 'think_enabled'")
+
+
+def downgrade() -> None:
+    # No-op. The setting is dead; restoring NULL rows wouldn't help.
+    pass