- Move inline DTOs to GmailDtos.cs, pure parse helpers to GmailParsing.cs
- Batch per-message existence checks in CreateSuggestedJob/RefreshLinkedThreads
- Remove redundant second pass in RelinkThread, reuse existing HashSet
- Replace ToListAsync+scan with FirstOrDefaultAsync for GmailReviewDecisions lookups
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
b4 of the multi-provider email roadmap. The manual/free-text correspondence
entry path already existed (CorrespondenceController.Create) -- this slice
was narrower than the roadmap wording suggests: tag every Correspondence row
with which provider it came from (gmail | manual today; microsoft | imap
once those providers grow an import-into-Correspondence path of their own),
not build a new endpoint.
- Correspondence.Provider (nullable string), reconciled via the existing
EnsureColumn pattern (SQLite + MySQL).
- Idempotent backfill: rows with an ExternalThreadId (historically only
ever written by Gmail import) get 'gmail'; everything else gets 'manual'.
- GmailController.ImportSingleMessageAsync now tags Provider = "gmail".
- CorrespondenceController.Create now tags Provider = "manual".
- Both write sites use a fixed literal, not request input -- no injection
surface introduced. Backfill SQL is static, no interpolation.
148/148 green (147 existing + 1 new CorrespondenceControllerTests; the
GmailController import test gained a Provider assertion in place).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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