feat: Add database indices to improve FTS3Agent performance

Add composite indices on (lastUpdate, assignment) for both Jobs and Operations tables in FTS3DB to optimize query performance. In LHCb these indices improve FTS3Agent cycle times from ~1400s to ~1000s.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: chrisburr

src/DIRAC/DataManagementSystem/DB/FTS3DB.py

@@ -16,6 +16,7 @@
     Enum,
     Float,
     ForeignKey,
+    Index,
     Integer,
     MetaData,
     SmallInteger,
@@ -85,6 +86,7 @@
     Column("error", String(2048)),
     Column("status", Enum(*FTS3Job.ALL_STATES), server_default=FTS3Job.INIT_STATE, index=True),
     Column("assignment", String(255), server_default=None),
+    Index("idx_jobs_lastupdate_assignment", "lastUpdate", "assignment"),
     mysql_engine="InnoDB",
 )
 
@@ -110,6 +112,7 @@
     Column("error", String(1024)),
     Column("type", String(255)),
     Column("assignment", String(255), server_default=None),
+    Index("idx_operations_lastupdate_assignment", "lastUpdate", "assignment"),
     mysql_engine="InnoDB",
 )