Merged in feature/mutable-metadata1 (pull request #221)

M1, M2 and M3 complete

* M1, M2 and M3 complete

* review changes

* docs

* docs

internal/database/queries/document.sql

@@ -61,7 +61,10 @@ RIGHT JOIN max_part mp ON p.part = mp.max_part_num
 GROUP BY p.part;
 
 -- name: GetDocumentEnriched :one
--- Retrieves a document with extended metadata including folder and filename
+-- Retrieves a document with extended metadata including folder and filename.
+-- Milestone 2.3a adds custom_schema_id and a correlated EXISTS probe into
+-- document_custom_metadata so GET /document/{id} can surface both fields
+-- without a follow-up query.
 SELECT
     d.id,
     d.clientId,
@@ -69,10 +72,27 @@ SELECT
     d.folderId,
     d.filename,
     d.originalPath,
-    d.file_size_bytes
+    d.file_size_bytes,
+    d.custom_schema_id,
+    EXISTS(
+        SELECT 1 FROM document_custom_metadata
+        WHERE document_id = d.id
+    ) AS has_custom_metadata
 FROM documents d
 WHERE d.id = $1;
 
+-- name: LockDocumentForLegacyExtractionWrite :one
+-- Milestone 2.5 parent-row lock acquired by CreateFieldExtraction as the
+-- FIRST DML statement inside its transaction. Serializes with AssignSchema
+-- (which takes FOR UPDATE on the same row) so the mutual-exclusivity
+-- invariant from plan Section 7.3 holds even if Trigger 2 is ever removed.
+-- Returns the current custom_schema_id so the caller can short-circuit to
+-- ErrMutualExclusivityViolation when the document is already bound.
+SELECT id, custom_schema_id
+FROM documents
+WHERE id = @document_id
+FOR UPDATE;
+
 -- name: CollectDocumentS3Paths :many
 -- @sqlc-vet-disable
 -- Collect S3 bucket+key from documentEntries before deleting, for logging orphaned S3 objects