feat(syslog): list patterns and add a ready-made one through MCP

Patterns could only be added on the Syslog page, so an agent asked to map
them on the NASes couldn't. list_syslog_patterns gives each pattern's metric
and the ready-made ones not added; add_syslog_preset (admin, audited) adds one.
The key-picking helper moves beside the patterns for both callers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/syslog/patterns.py

@@ -87,6 +87,16 @@ def key_for(name: str) -> str:
     return _KEY.sub("_", name.lower()).strip("_")[:48] or "pattern"
 
 
+async def free_key(db, wanted: str) -> str:
+    """`wanted`, or that with a number where another pattern has it."""
+    taken = set((await db.execute(select(SyslogPattern.key))).scalars())
+    key, n = wanted, 2
+    while key in taken:
+        key = f"{wanted[:44]}_{n}"
+        n += 1
+    return key
+
+
 def parse_form(form) -> dict:
     """A pattern's fields from its form, or PatternError saying what to fix."""
     name = (form.get("name") or "").strip()[:100]