Expose usedns, date_pattern, and prefregex in jail config UI

- Add use_dns and prefregex fields to JailConfig model (backend + frontend types)
- Add prefregex to JailConfigUpdate; validate as regex before writing
- Fetch usedns and prefregex in get_jail_config via asyncio.gather
- Write usedns and prefregex in update_jail_config
- ConfigPage JailAccordionPanel: editable date_pattern input, dns_mode
  Select dropdown (yes/warn/no/raw), and prefregex input
- 8 new service unit tests + 3 new router integration tests
- 628 tests pass; 85% line coverage; ruff/mypy/tsc/eslint clean

backend/app/models/config.py

@@ -79,6 +79,8 @@ class JailConfig(BaseModel):
     date_pattern: str | None = Field(default=None, description="Custom date pattern for log parsing.")
     log_encoding: str = Field(default="UTF-8", description="Log file encoding.")
     backend: str = Field(default="polling", description="Log monitoring backend.")
+    use_dns: str = Field(default="warn", description="DNS lookup mode: yes | warn | no | raw.")
+    prefregex: str = Field(default="", description="Prefix regex prepended to every failregex; empty means disabled.")
     actions: list[str] = Field(default_factory=list, description="Names of actions attached to this jail.")
     bantime_escalation: BantimeEscalation | None = Field(
         default=None,
@@ -113,8 +115,9 @@ class JailConfigUpdate(BaseModel):
     find_time: int | None = Field(default=None, ge=1)
     fail_regex: list[str] | None = Field(default=None, description="Failure detection regex patterns.")
     ignore_regex: list[str] | None = Field(default=None)
+    prefregex: str | None = Field(default=None, description="Prefix regex; None = skip, '' = clear, non-empty = set.")
     date_pattern: str | None = Field(default=None)
-    dns_mode: str | None = Field(default=None, description="DNS lookup mode: raw | warn | no.")
+    dns_mode: str | None = Field(default=None, description="DNS lookup mode: yes | warn | no | raw.")
     enabled: bool | None = Field(default=None)
     bantime_escalation: BantimeEscalationUpdate | None = Field(
         default=None,