refactor(embedding-cache): rename index cache to embedding cache

The cached payload is the indexing pipeline's embeddings (markdown is
chunked then embedded), so "embedding cache" names the expensive output
directly and removes the "index" ambiguity (DB index vs vector index vs
indexing phase). Renames the service, settings, eligibility, eviction
task, metrics, config flags (INDEX_CACHE_* -> EMBEDDING_CACHE_*), object
prefix, and the table (index_cache_embedding_sets -> embedding_cache_sets)
with its constraint and indexes. Migration 161 renamed accordingly.

surfsense_backend/tests/unit/indexing_pipeline/cache/test_eligibility.py

@@ -1,28 +1,28 @@
-from app.indexing_pipeline.cache.eligibility import is_index_cacheable
+from app.indexing_pipeline.cache.eligibility import is_embedding_cacheable
 
 
 def test_disabled_cache_is_never_cacheable():
-    assert not is_index_cacheable(
+    assert not is_embedding_cacheable(
         cache_enabled=False, embedding_model="m", embedding_dim=384
     )
 
 
 def test_missing_model_is_not_cacheable():
-    assert not is_index_cacheable(
+    assert not is_embedding_cacheable(
         cache_enabled=True, embedding_model=None, embedding_dim=384
     )
 
 
 def test_missing_dimension_is_not_cacheable():
-    assert not is_index_cacheable(
+    assert not is_embedding_cacheable(
         cache_enabled=True, embedding_model="m", embedding_dim=None
     )
-    assert not is_index_cacheable(
+    assert not is_embedding_cacheable(
         cache_enabled=True, embedding_model="m", embedding_dim=0
     )
 
 
 def test_enabled_with_model_and_dim_is_cacheable():
-    assert is_index_cacheable(
+    assert is_embedding_cacheable(
         cache_enabled=True, embedding_model="m", embedding_dim=384
     )