fix(orchestrator): drop topology_deckies FK on event src/dst columns

Once the orchestrator started seeing fleet + SWARM shard sources via
list_running_deckies (a844148), every event row landing on a fleet decky
broke the FK to topology_deckies — the column now carries opaque ids
("local:omega-decky" for fleet, "host_uuid:decky_name" for shards) that
will never match topology_deckies.uuid.

Symptom on the operator's mothership:
  IntegrityError 1452 — orchestrator_events_ibfk_2 FK violated on every
  tick once the reconciler populated fleet_deckies.

Index on dst_decky_uuid is preserved (the dashboard reads
"events for this decky" frequently); only the FK is removed.  Keeps
data integrity loose by design — events are append-only history that
should outlive the deckies they reference.

Existing MySQL deployments need the FK dropped manually:
  ALTER TABLE orchestrator_events
    DROP FOREIGN KEY orchestrator_events_ibfk_2,
    DROP FOREIGN KEY orchestrator_events_ibfk_1;

SQLite users are unaffected — SQLite doesn't enforce FKs by default.

decnet/web/db/models/orchestrator.py

@@ -41,12 +41,14 @@ class OrchestratorEvent(SQLModel, table=True):
     kind: str = Field(index=True, max_length=16)              # traffic|file
     protocol: str = Field(index=True, max_length=16)          # ssh for MVP
     action: str = Field(max_length=64)                         # exec:uptime|file:create|...
-    src_decky_uuid: Optional[str] = Field(
-        default=None, foreign_key="topology_deckies.uuid", index=True
-    )
-    dst_decky_uuid: str = Field(
-        foreign_key="topology_deckies.uuid", index=True
-    )
+    # No FK to topology_deckies: dst/src may be a TopologyDecky.uuid
+    # (MazeNET source), a "host_uuid:name" composite (fleet / SWARM shard
+    # sources), or — for retired deckies — a row that's already gone.  The
+    # column is an opaque identifier matching whatever
+    # ``BaseRepository.list_running_deckies`` emits in its ``uuid`` field.
+    # Index is kept; the FK was misleading and broke fleet-source events.
+    src_decky_uuid: Optional[str] = Field(default=None, index=True)
+    dst_decky_uuid: str = Field(index=True)
     success: bool = Field(default=False, index=True)
     payload: str = Field(
         sa_column=Column("payload", Text, nullable=False, default="{}")