Add version footer, task history UI, and note version retention policy

- App.vue: quiet version footer (fetches /api/version, 0.45 opacity)
- TaskEditorView: History button + HistoryPanel integration (mirrors NoteEditorView)
- note_versions.py: raise MAX_VERSIONS 20→50; add 5-min minimum interval gate
  to prevent autosave exhausting history slots

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/note_versions.py

@@ -1,15 +1,39 @@
+from datetime import datetime, timezone
+
 from sqlalchemy import select, text
 
 from fabledassistant.models import async_session
 from fabledassistant.models.note_version import NoteVersion
 
-MAX_VERSIONS = 20
+# Maximum snapshots retained per note.
+MAX_VERSIONS = 50
+
+# Minimum seconds between snapshots. Prevents autosave from consuming all
+# slots — with autosave at 60 s intervals, a 5-minute gate means at most
+# one snapshot per editing session segment rather than one per save tick.
+MIN_VERSION_INTERVAL_SECONDS = 300
 
 
 async def create_version(
     user_id: int, note_id: int, body: str, title: str, tags: list[str] | None = None
-) -> NoteVersion:
+) -> NoteVersion | None:
     async with async_session() as session:
+        # Skip if a recent snapshot already exists within the minimum interval.
+        recent = await session.execute(
+            select(NoteVersion.created_at)
+            .where(NoteVersion.note_id == note_id, NoteVersion.user_id == user_id)
+            .order_by(NoteVersion.created_at.desc())
+            .limit(1)
+        )
+        last_at = recent.scalar_one_or_none()
+        if last_at is not None:
+            # created_at is stored with timezone; ensure comparison is tz-aware
+            if last_at.tzinfo is None:
+                last_at = last_at.replace(tzinfo=timezone.utc)
+            age = (datetime.now(timezone.utc) - last_at).total_seconds()
+            if age < MIN_VERSION_INTERVAL_SECONDS:
+                return None
+
         version = NoteVersion(note_id=note_id, user_id=user_id, body=body, title=title, tags=tags or [])
         session.add(version)
         await session.commit()