Make the embedding model swappable via env for A/B testing

Trialing Qwen3-Embedding-0.6B (1024-dim, instruct-prefixed queries)
against nomic required code changes at every hardcoded seam; now it's a
config flip plus a reembed_embeddings run.

- EMBEDDING_DIM env (default 768) replaces every hardcoded dim check:
  daily summary / calendar / search / location DAOs, Ollama batch
  validation, reembed_embeddings
- entities gains the dim guard it never had — a wrong-dim vector
  silently kills dedup/recall (cosine over mismatched lengths is 0),
  so store None and warn instead
- embed_query / embed_document split with EMBED_QUERY_PREFIX /
  EMBED_DOCUMENT_PREFIX (literal \n expanded): retrieval models treat
  the two sides differently — nomic wants search_query:/search_document:,
  Qwen3 wants Instruct:...\nQuery: on queries only. All query-side
  call sites and all corpus writers now declare their side.
- document the contract in CLAUDE.md: change the model or any of these
  vars → re-run reembed_embeddings or search is garbage

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/database/calendar_dao.rs

@@ -222,11 +222,12 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             // Validate embedding dimensions if provided
             if let Some(ref emb) = event.embedding
-                && emb.len() != 768
+                && emb.len() != crate::ai::embedding_dim()
             {
                 return Err(anyhow::anyhow!(
-                    "Invalid embedding dimensions: {} (expected 768)",
-                    emb.len()
+                    "Invalid embedding dimensions: {} (expected {})",
+                    emb.len(),
+                    crate::ai::embedding_dim()
                 ));
             }
 
@@ -293,7 +294,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
                 for event in events {
                     // Validate embedding if provided
                     if let Some(ref emb) = event.embedding
-                        && emb.len() != 768
+                        && emb.len() != crate::ai::embedding_dim()
                     {
                         log::warn!(
                             "Skipping event with invalid embedding dimensions: {}",
@@ -385,10 +386,11 @@ impl CalendarEventDao for SqliteCalendarEventDao {
         trace_db_call(context, "query", "find_similar_events", |_span| {
             let mut conn = self.connection.lock().expect("Unable to get CalendarEventDao");
 
-            if query_embedding.len() != 768 {
+            if query_embedding.len() != crate::ai::embedding_dim() {
                 return Err(anyhow::anyhow!(
-                    "Invalid query embedding dimensions: {} (expected 768)",
-                    query_embedding.len()
+                    "Invalid query embedding dimensions: {} (expected {})",
+                    query_embedding.len(),
+                    crate::ai::embedding_dim()
                 ));
             }
 
@@ -461,10 +463,11 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             // Step 2: If query embedding provided, rank by semantic similarity
             if let Some(query_emb) = query_embedding {
-                if query_emb.len() != 768 {
+                if query_emb.len() != crate::ai::embedding_dim() {
                     return Err(anyhow::anyhow!(
-                        "Invalid query embedding dimensions: {} (expected 768)",
-                        query_emb.len()
+                        "Invalid query embedding dimensions: {} (expected {})",
+                        query_emb.len(),
+                        crate::ai::embedding_dim()
                     ));
                 }