feat: add optional mood to thoughts with custom moods support

Mood is an optional label+emoji string (e.g. "relaxed 😌") on thoughts.
Users can define up to 8 custom moods in profile settings.
Mood federates via AP Note JSON and displays on thought cards.

crates/adapters/postgres/migrations/021_thought_mood_and_custom_moods.sql

@@ -0,0 +1,2 @@
+ALTER TABLE thoughts ADD COLUMN IF NOT EXISTS mood VARCHAR(64);
+ALTER TABLE users ADD COLUMN IF NOT EXISTS custom_moods JSONB DEFAULT '[]'::jsonb;