fix: match Amazon card charges and anchor Monarch discovery to the order

[eshaffer321]

Fixes two Amazon sync errors and a latent wrong-match bug found while tracing them.

What was wrong

Order	Symptom	Cause
112-3421348-5157859	Error: could not find all transactions: expected 2, found 2	Only $19.48 of the $13.77 + $19.48 charges had posted. The handler counted nil placeholders in MultiMatchResult.Matches as matches, and reported a partial match as an error.
112-7815140-3755432	Error: bank charges ($59.36) exceed expected ($55.31)	Amazon charged the card $59.36; a $4.05 discount on the order wasn't applied to the charge. The handler never tried the reported card charge, which was in Monarch, and only searched for transactions summing to the order total.
112-7815140-3755432, synced alone	Matched $25.44 + $29.87, charges from two other orders	FindSubsetByTotal accepts any subset that sums to the total. The only thing preventing this in the full run was which orders happened to claim those transactions first.

Changes

• Count only non-nil matches, and skip a partial multi-charge match as payment pending rather than an error.
• When card charges meet or exceed the expected amount, match them against Monarch directly and allocate using the charged amount.
• Add Matcher.FindSubsetByTotalIncluding: discovery must include at least one charge Amazon reported for the order. Walmart and the simple handler keep the old FindSubsetByTotal behavior.

Testing

• Failing tests written first, using the real order amounts (see docs/bug-fixes.md); each failed against main for the reason above.
• go test ./... -race and golangci-lint pass. Coverage: matcher 83.4% → 91.0%, handlers 74.0% → 75.0%.
• itemize amazon -account wife -dry-run: Errors 2 → 0. 112-3421348-5157859 is pending (expected=2 found=1), and 112-7815140-3755432 matches the $59.36 transaction.

[codecov]

Codecov Report

❌ Patch coverage is 84.53608% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.37%. Comparing base (82e976b) to head (8d654f4).

Files with missing lines	Patch %	Lines
internal/application/sync/handlers/amazon.go	83.11%	10 Missing and 3 partials ⚠️
internal/domain/matcher/subset.go	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   64.99%   65.37%   +0.38%     
==========================================
  Files          51       51              
  Lines        6990     7036      +46     
==========================================
+ Hits         4543     4600      +57     
+ Misses       2108     2094      -14     
- Partials      339      342       +3     

Flag	Coverage Δ
unittests	65.37% <84.53%> (+0.38%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/domain/matcher/subset.go	81.81% <90.00%> (+22.99%)	⬆️
internal/application/sync/handlers/amazon.go	70.71% <83.11%> (+4.44%)	⬆️

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