feat: configure FastEmbed model in config and admin, with restart to apply

Expose fastembed_model in config and a model dropdown on Admin → Models.
Persist dimension from the chosen model, require restart to load it, and
align legacy OpenAI default settings so fresh local-embedding installs
start cleanly.

common/db/migrations/20260604_000001_fastembed_default_embedding_model.surql

@@ -0,0 +1,8 @@
+-- Align persisted embedding settings when FastEmbed is the recorded backend but the model
+-- name is still the OpenAI migration default (invalid for FastEmbed `from_str`).
+
+UPDATE system_settings:current SET
+    embedding_model = 'Xenova/bge-small-en-v1.5',
+    embedding_dimensions = 384
+WHERE embedding_backend = 'fastembed'
+    AND embedding_model = 'text-embedding-3-small';