refactor(orchestrator): collapse decnet-emailgen.service into orchestrator

Stage 5 of the realism migration. Email generation is no longer a
separate worker / systemd unit / CLI subcommand — the orchestrator's
single tick loop covers SSH traffic, file plants, and email drops.
Going from 21 services to 20.

Worker:
- _one_tick rolls between traffic / file / email (45/45/10 weights).
  The 10% email weight at a 60s orchestrator interval produces ~one
  email per 10 minutes, close to the pre-collapse 5-minute cadence.
- get_driver_for(action) (stage 4) handles SSH vs Email dispatch.
- Quiet branches fall through so a (decky-set, persona-pool,
  mail-decky) shape that silences one branch doesn't waste the tick.
- Periodic prune covers both orchestrator_events and
  orchestrator_emails tables.

Deletions:
- deploy/decnet-emailgen.service.j2
- decnet/orchestrator/emailgen/worker.py
- decnet/cli/emailgen.py
- tests/orchestrator/emailgen/test_worker_integration.py

Renames (history-preserving):
- decnet/web/router/emailgen/ -> decnet/web/router/realism/
- tests/api/emailgen/        -> tests/api/realism/
- tests/cli/test_emailgen_*  -> tests/cli/test_realism_*

Public surface changes (clean break, pre-v1):
- API URL /api/v1/emailgen/personas -> /api/v1/realism/personas
- CLI `decnet emailgen import-personas` -> `decnet realism
  import-personas`. `decnet emailgen run` is gone — the orchestrator
  covers it.
- gating.py: emailgen master-only group replaced by realism.
- decnet-orchestrator.service.j2: DECNET_REALISM_* env block added.
- decnet.target: decnet-emailgen.service entry removed.
- frontend: PersonaGeneration.tsx fetches /realism/personas.

decnet/cli/gating.py

@@ -32,7 +32,7 @@ MASTER_ONLY_COMMANDS: frozenset[str] = frozenset({
     "db-reset", "init", "webhook", "clusterer", "campaign-clusterer",
 })
 MASTER_ONLY_GROUPS: frozenset[str] = frozenset(
-    {"swarm", "topology", "geoip", "emailgen"}
+    {"swarm", "topology", "geoip", "realism"}
 )