Parent epic: #2385

Scope Change

Originally scoped as generics migration. After adversarial critique review, the generics migration is deferred to backlog (P4) because:

1. sqlx::migrate! is monomorphic — requires #[cfg] per backend
2. Trait bounds explosion (5 HRTB bounds in FullDriver already)
3. The #[cfg] type alias approach is pragmatically correct for pre-v1.0

This issue is repurposed for concrete code quality and performance fixes.

Changes

Pool Configuration (P2)

• SQLite pool: Add min_connections(2) and acquire_timeout(5s) to pool.rs:78-82
• Postgres pool: Fix connect_postgres to use max_connections correctly — currently pool_size field is silently ignored (pool.rs:99)
• Rename/consolidate DbConfig fields: pool_size → remove, use max_connections only

sql! Macro (P2)

• Replace Box::leak() with LazyLock per call site in postgres variant (lib.rs:102-111):

  macro_rules! sql {
      ($query:expr) => {{
          static REWRITTEN: std::sync::LazyLock<String> =
              std::sync::LazyLock::new(|| $crate::rewrite_placeholders($query));
          REWRITTEN.as_str()
      }};
  }

• Fix the misleading doc comment that claims LazyLock is already used

Error Consolidation (P2)

• Merge MemoryError::Sqlx(sqlx::Error) and MemoryError::Db(DbError) into single MemoryError::Db
• Remove backward-compat aliases: SqliteStore (store/mod.rs:32), SqliteVectorStore (db_vector_store.rs:24)

Edge Upsert Optimization (P2)

• Replace SELECT+UPDATE/INSERT in insert_edge_typed (graph/store/mod.rs:413-437) with single INSERT ... ON CONFLICT DO UPDATE ... RETURNING id
• Halves DB round-trips per graph extraction pass

Error Handling (P2)

• Fix get_vectors (embedding_store.rs:372) — .unwrap_or_default() silently swallows all DB errors. Propagate errors properly.

Acceptance Criteria

• SQLite pool has min_connections + acquire_timeout
• Postgres pool respects max_connections config
• sql! macro on postgres allocates once per call site (add a test)
• No MemoryError::Sqlx variant
• No SqliteStore / SqliteVectorStore aliases
• insert_edge_typed uses single upsert query
• get_vectors propagates errors
• All tests pass