Add AI-suggested editable document title

AI now returns a short descriptive title per document (e.g. "ACME Corp
Invoice April 2026"). Title is stored in a new documents.title column
(migration 0002), shown in the row header instead of the raw filename,
and editable inline via PATCH /documents/{id}/title. Filename is shown
as a subtitle when a title exists.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/doc-service/app/services/prompts.py

@@ -5,6 +5,7 @@ SYSTEM_PROMPT = (
 )
 
 USER_PROMPT_TEMPLATE = """Analyze the following document text and return a JSON object with exactly these keys:
+title (a short, descriptive human-readable title for this document, e.g. "ACME Corp Invoice April 2026", "Office Supplies Receipt", "Q1 Flower Delivery Order"),
 document_type (one of: invoice, bill, receipt, order, expense, revenue, unknown),
 total_amount (string or null),
 currency (string or null),