Implement hybrid approach for balance assertions

Balance assertions now use a hybrid architecture where Beancount is the source
of truth for validation, while Castle stores metadata for UI convenience.

Backend changes:
- Add format_balance() function to beancount_format.py for formatting balance directives
- Update POST /api/v1/assertions to write balance directive to Beancount first (via Fava)
- Store metadata in Castle DB (created_by, tolerance, notes) for UI features
- Validate assertions immediately by querying Fava for actual balance

Frontend changes:
- Update dialog description to explain Beancount validation
- Update button tooltip to clarify balance assertions are written to Beancount
- Update empty state message to mention Beancount checkpoints

Benefits:
- Single source of truth (Beancount ledger file)
- Automatic validation by Beancount
- Best of both worlds: robust validation + friendly UI

See misc-docs/BALANCE-ASSERTIONS-HYBRID-APPROACH.md for full documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

templates/castle/index.html

@@ -520,7 +520,7 @@
             icon="add"
             label="Create Assertion"
           >
-            <q-tooltip>Create a new balance assertion for reconciliation</q-tooltip>
+            <q-tooltip>Write a balance assertion to Beancount ledger for automatic validation</q-tooltip>
           </q-btn>
         </div>
 
@@ -629,7 +629,7 @@
 
         <!-- No assertions message -->
         <div v-if="balanceAssertions.length === 0" class="text-center text-grey q-pa-md">
-          No balance assertions yet. Create one to verify your accounting accuracy.
+          No balance assertions yet. Create one to add checkpoints to your Beancount ledger and verify accounting accuracy.
         </div>
       </q-card-section>
     </q-card>
@@ -1185,7 +1185,10 @@
       <div class="text-h6 q-mb-md">Create Balance Assertion</div>
 
       <div class="text-caption text-grey q-mb-md">
-        Balance assertions help you verify accounting accuracy by checking if an account's actual balance matches your expected balance. If the assertion fails, you'll be alerted to investigate the discrepancy.
+        Balance assertions are written to your Beancount ledger and validated automatically by Beancount.
+        This verifies that an account's actual balance matches your expected balance at a specific date.
+        If the assertion fails, Beancount will alert you to investigate the discrepancy. Castle stores
+        metadata (tolerance, notes) for your convenience.
       </div>
 
       <q-select