fix(db): match digest subscriptions case-insensitively by login

[galuis116]

Summary

Fixes #1523.

upsertDigestSubscription and listDigestSubscriptionsForLogin keyed digest subscriptions on the raw login without case-normalizing it — even though the email is already lowercased, and every sibling subscription/identity path (notification subscriptions, issue-watch, notification deliveries, official-miner detections) lowercases login on both write and read. GitHub logins are case-insensitive, so a subscriber stored as "Foo" was missed on a "foo" lookup, and the [login, email] conflict target accumulated case-variant duplicate rows.

What changed

• upsertDigestSubscription now stores login: input.login.toLowerCase(), and listDigestSubscriptionsForLogin looks up with login.toLowerCase() — making storage and lookup consistent and the [login, email] dedup effective, exactly like the sibling subscription paths.

Scope

• Conventional Commit title.
• Focused; no unrelated changes.
• Follows CONTRIBUTING.md; no site//CNAME/VitePress.
• Linked issue ([Bug]: digest subscriptions are keyed by case-sensitive login (missed lookups + case-variant duplicate rows) #1523).

Validation

• git diff --check
• npm run typecheck
• npm run test:coverage locally (4619 passed); both changed lines are covered by a new case-insensitive + dedup test.
• npm run test:workers
• npm audit --audit-level=moderate
• New unit test: mixed-case subscribe + cross-case lookup resolves; re-subscribe under another casing updates the single row (no duplicate).

If any required check was skipped, explain why:

• OpenAPI/types/migrations unchanged — this normalizes an input before existing queries; the schema and DigestSubscriptionRecord shape are identical.

Safety

• No secrets, wallets, hotkeys, PATs, trust scores, or private evidence exposed.
• Public text sanitized and low-noise.
• No auth/CORS/session change.
• API/OpenAPI/MCP behavior unchanged.
• No UI changes.
• No changelog edit.

Notes

• Confined to two functions in src/db/repositories.ts; mirrors the existing case-insensitive handling in the notification/issue-watch subscription paths.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.45%. Comparing base (6b1389b) to head (0f5bc00).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1524   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files         195      195           
  Lines       21121    21121           
  Branches     7637     7637           
=======================================
  Hits        20161    20161           
  Misses        383      383           
  Partials      577      577           

Files with missing lines	Coverage Δ
src/db/repositories.ts	96.12% <100.00%> (ø)

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