Management UI: corpus delete, process panel, activity feed, diagnostics

- DELETE /api/training/corpus/{id} + delete button on training page,
  with confirmation dialog and recompute hint
- /api/system/tasks + floating process panel (bottom-left) showing
  active background tasks with live 3s polling
- /api/system/recent-activity derives a feed from cases, style_corpus,
  and last style_patterns run; sidebar on home page renders with
  relative timestamps
- /api/system/diagnostics + /#/diagnostics page showing DB health,
  row counts per table, active tasks, stuck documents (>10 min),
  failed extractions
- Cosmetic: signature phrase headline now prefers clean phrases over
  bracket-heavy templates for display

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/db.py

@@ -794,6 +794,51 @@ async def add_to_style_corpus(
     return corpus_id
 
 
+async def delete_from_style_corpus(corpus_id: UUID) -> dict:
+    """Remove a decision from style_corpus + related documents (cascades chunks).
+
+    Also tries to delete the [קורפוס] document associated by title match,
+    since the current training pipeline inserts style_corpus with document_id=NULL.
+    """
+    pool = await get_pool()
+    async with pool.acquire() as conn:
+        async with conn.transaction():
+            row = await conn.fetchrow(
+                "DELETE FROM style_corpus WHERE id = $1 "
+                "RETURNING decision_number, document_id",
+                corpus_id,
+            )
+            if not row:
+                return {"deleted": False, "reason": "not found"}
+
+            docs_deleted = 0
+            if row["document_id"]:
+                await conn.execute(
+                    "DELETE FROM documents WHERE id = $1", row["document_id"]
+                )
+                docs_deleted = 1
+            else:
+                # Best-effort: match a [קורפוס] document by the decision_number
+                # in its title. Only for single, unambiguous matches.
+                if row["decision_number"]:
+                    docs = await conn.fetch(
+                        "SELECT id FROM documents "
+                        "WHERE case_id IS NULL AND title LIKE $1",
+                        f"%{row['decision_number']}%",
+                    )
+                    if len(docs) == 1:
+                        await conn.execute(
+                            "DELETE FROM documents WHERE id = $1", docs[0]["id"]
+                        )
+                        docs_deleted = 1
+
+    return {
+        "deleted": True,
+        "decision_number": row["decision_number"],
+        "docs_deleted": docs_deleted,
+    }
+
+
 async def get_style_patterns(pattern_type: str | None = None) -> list[dict]:
     pool = await get_pool()
     async with pool.acquire() as conn: