Zerodha funds: availablecash equals opening balance; expose live_balance + openingbalance for exact total/free margin

[arsalanansari17]

Summary

For Zerodha, /api/v1/funds cannot give a caller either the live cash balance or the exact total/free margin once positions are open. Two small changes to broker/zerodha/api/funds.py would fix both:

1. availablecash = Kite's available.live_balance (real intraday cash), and
2. a new, additive field openingbalance = Kite's available.opening_balance.

Current behaviour

availablecash is computed as net + debits - collateral (the #1582 fix). Kite's margins response satisfies net = opening_balance + collateral - debits, so this simplifies to opening_balance: the static start-of-day balance, not the cash available now. It doesn't move as margin is used or cash is spent, which was the original complaint in #1582.

Switching availablecash to live_balance alone doesn't fix it either, because of how Zerodha blocks margin. Zerodha takes margin roughly 50:50 from cash and collateral (and credits option premium received to cash). So live_balance drops by only the cash share of the margin in use, and a caller can no longer rebuild the account's total or free margin from the five returned fields:

Illustrative account, iron fly open	Value
Opening balance	4,00,000
Collateral (pledged holdings)	5,00,000
Used margin (SPAN + exposure − premium)	1,30,000
Kite "Available cash" (live_balance)	~3,43,000 (dropped by only ~57K)
Kite "Available margin"	7,70,000 = 4,00,000 + 5,00,000 − 1,30,000
Total rebuilt as live_balance + collateral + debits	~9,73,000 vs the real 9,00,000 (counts the collateral half twice)
Free rebuilt from that	~8,43,000 vs Kite's 7,70,000

We checked this against the Kite Funds page on a live account: opening balance + collateral − used margin matched Kite's "Available margin" to the rupee, and the cash drop equalled 50% of (SPAN + exposure) minus the option premium received.

Proposed change (Zerodha only, get_margin_data)

"availablecash": sum(available.live_balance for equity, commodity),   # real cash now
# additive, omitted if Kite leaves it out:
"openingbalance": sum(available.opening_balance for equity, commodity),

Callers then get Kite's own numbers:

• live cash = availablecash
• total capital = openingbalance + collateral (static intraday)
• available margin = openingbalance + collateral - utiliseddebits

The openingbalance field is purely additive. The availablecash change does alter what the Dashboard / Telegram /funds show during the day, which is why this is an issue first rather than a PR. Happy to open a PR with both changes (we've run them on our own fork) if this direction is acceptable.

Related: #1582