feat: foreign flow from IPOT by default (--source ipot|idx)

`idx stocks flow` now reads IPOT by default, which needs nothing installed:
per session, the all-investor broker summary gives the value traded and
the foreign one gives foreign buying (its footer), the foreign net and
selling (buying less the net), across all boards, through the broker
summary cache. IPOT's foreign net matched IDX's estimate within 0.3 B on
BBCA 2026-10-12 to 16.

`--source idx` (and `source` on get_foreign_flow) keeps IDX's trading
summary: exact share counts, close and the non-regular share, behind
curl-impersonate and Cloudflare.

- ForeignFlowDay gains foreign_buy_value, foreign_sell_value and
  foreign_share_bps for both sources (estimated at the session average
  for IDX); ForeignFlow gains source, the value totals and rounded.
- The table lays out each source's figures; the live smoke runs both.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V3syn2fN2LNo5pvTSqc6eh

web/src/content/docs/cli/reference.md

@@ -433,7 +433,7 @@ Examples:
 ### `idx stocks flow`
 
 ```text
-Foreign buying and selling of a stock, session by session, from IDX's daily trading summary: foreign buy, sell and net volume, the net value at the session's average price, and the share of value traded outside the regular board. End of day. Each session is one request to idx.co.id, which needs curl-impersonate.
+Foreign buying and selling of a stock, session by session, end of day. By default from Indo Premier (IPOT): foreign buy, sell and net value across all boards and foreign investors' share of trading, two plain requests per session. `--source idx` reads IDX's own trading summary instead: exact foreign share counts, the close and the share traded outside the regular board, one request per session to idx.co.id, which needs curl-impersonate and a network IDX's Cloudflare lets through.
 
 Usage: idx stocks flow [OPTIONS] <SYMBOL>
 
@@ -462,6 +462,15 @@ Options:
       --to <TO>
           Last session of the range, YYYY-MM-DD. Default: today
 
+      --source <SOURCE>
+          Where the figures come from
+
+          Possible values:
+          - ipot: Indo Premier's broker summary: foreign buy, sell and net value across all boards, two plain requests per session. Figures of a million and more are rounded to one decimal
+          - idx:  IDX's own trading summary: exact foreign share counts, close and the non-regular share, regular board. Needs curl-impersonate and a network IDX's Cloudflare lets through
+
+          [default: ipot]
+
   -v, --verbose...
           Increase verbosity (-v, -vv)
 
@@ -478,7 +487,8 @@ Examples:
   idx stocks flow BBCA
   idx stocks flow BBCA --days 20
   idx stocks flow BBCA --from 2026-10-01 --to 2026-10-16
-  idx -o json stocks flow BBCA | jq '.foreign_net'
+  idx stocks flow BBCA --source idx
+  idx -o json stocks flow BBCA | jq '.foreign_net_value'
 ```
 
 ### `idx stocks profile`