Ensure live provider checks reach the provider

Accept Docker Compose's exact string representation of the fixed 1024-dimensional embedding contract while rejecting every widened or ambiguous value. Entry-level smoke tests now exercise the same environment shape used by the container.

Constraint: The current local Bailian credential still returns 401 for all three capabilities and is not stored in Git.
Rejected: Change the field to an unconstrained integer | would allow a runtime dimension that disagrees with vector(1024).
Confidence: high
Scope-risk: narrow
Directive: Keep the database vector dimension and provider dimension locked together through an ADR-backed migration.
Tested: make verify; 75 backend tests; 14 frontend tests; rebuilt provider-smoke image; normal Compose smoke reached all three provider endpoints.
Not-tested: Successful live provider response remains blocked by external authentication.

backend/app/core/config.py

@@ -71,6 +71,16 @@ class Settings(BaseSettings):
     def normalize_base_url(cls, value: str) -> str:
         return value.rstrip("/")
 
+    @field_validator("embedding_dimension", mode="before")
+    @classmethod
+    def parse_embedding_dimension(cls, value: object) -> int:
+        """Accept Compose's string representation without widening the contract."""
+        if value == "1024" or (
+            isinstance(value, int) and not isinstance(value, bool) and value == 1024
+        ):
+            return 1024
+        raise ValueError("EMBEDDING_DIMENSION must be exactly 1024")
+
     @model_validator(mode="after")
     def validate_rag_limits(self) -> Self:
         if self.chunk_overlap_tokens >= self.chunk_target_tokens: