Updates to reduce firestore calls to try and stay in free tier

### Firestore read reductions

**1. `doc_get_cached()` in `firestore.py` — new 5-min TTL cache**
One place, benefits everything. System and node config documents almost never change during a monitoring session.

**2. System doc: 4 reads → 1 per call**
| Before | After |
|---|---|
| `upload.py` — `doc_get("systems")` for ai_flags | `doc_get_cached` |
| `transcription.py` — `get_vocabulary()` → `doc_get("systems")` | cache hit |
| `intelligence.py` — `get_vocabulary()` → `doc_get("systems")` | cache hit |
| `intelligence.py` — `doc_get("systems")` again for ten_codes | eliminated (reads same cached doc) |

**3. Node doc: cached in `_on_call_start` and `intelligence.py`**
The node is read every call event to get `assigned_system_id` and lat/lon for geocoding. Both now use the cache — node assignments and positions essentially never change at runtime.

**4. Node sweeper: 30s → 90s interval**
The sweeper was doing a full node collection scan 3× more often than necessary — the offline threshold is already 90s. Cuts sweeper reads by 66%.

**5. Vocabulary induction: scans all-time calls → last 7 days**
Previously fetched every ended call for a system (could be thousands). Now scoped to the last 7 days.

> **Note:** The vocabulary induction query `(system_id == X, ended_at >= cutoff)` needs a Firestore
> composite index on `(system_id ASC, ended_at ASC)`. When the induction loop first fires it will log
> an error with a Firebase Console link to create it in one click.

drb-c2-core/app/internal/firestore.py

@@ -1,4 +1,5 @@
 import asyncio
+import time as _time
 from typing import Optional, Any
 import firebase_admin
 from firebase_admin import credentials, firestore as fs
@@ -6,6 +7,12 @@ from google.cloud.firestore_v1.base_query import FieldFilter
 from app.config import settings
 from app.internal.logger import logger
 
+# ---------------------------------------------------------------------------
+# In-memory TTL cache for rarely-changing documents (systems, nodes config)
+# ---------------------------------------------------------------------------
+# Key: "collection/doc_id"  →  (expires_at_monotonic, data_or_None)
+_doc_cache: dict[str, tuple[float, Optional[dict]]] = {}
+
 
 def _init_firebase():
     if firebase_admin._apps:
@@ -79,3 +86,19 @@ async def collection_where(
 async def doc_delete(collection: str, doc_id: str) -> None:
     ref = db.collection(collection).document(doc_id)
     await asyncio.to_thread(ref.delete)
+
+
+async def doc_get_cached(collection: str, doc_id: str, ttl: float = 300.0) -> Optional[dict]:
+    """
+    Like doc_get but backed by a short-lived in-memory TTL cache.
+    Use for documents that change rarely (systems config, node assignments).
+    Default TTL is 5 minutes — a write will be visible within that window.
+    """
+    key = f"{collection}/{doc_id}"
+    now = _time.monotonic()
+    entry = _doc_cache.get(key)
+    if entry and now < entry[0]:
+        return entry[1]
+    data = await doc_get(collection, doc_id)
+    _doc_cache[key] = (now + ttl, data)
+    return data