No shared tsconfig base: five packages drift, and workers is the least-checked

[InfinityBowman]

There is no shared TypeScript config. Four packages each restate their compiler options from scratch, and they have already drifted apart.

Current drift

Package	Flags set beyond strict
web	noUnusedLocals, noUnusedParameters, noFallthroughCasesInSwitch, noUncheckedSideEffectImports, verbatimModuleSyntax: false
workers	forceConsistentCasingInFileNames only
shared	noUnusedLocals, noUnusedParameters, noImplicitReturns, noFallthroughCasesInSwitch, forceConsistentCasingInFileNames
db	same as shared

No package uses extends. Notable consequences:

• packages/workers is the weakest-checked package in the repo -- no noUnusedLocals, no noUnusedParameters, no noImplicitReturns, no noFallthroughCasesInSwitch. That is the backend.
• noImplicitReturns is on in shared and db, off in web and workers.
• Nothing has noUncheckedIndexedAccess or exactOptionalPropertyTypes.

Why now

This is a prerequisite for #774. Turning on noUncheckedIndexedAccess per package means editing four files that will drift again; doing it once in a base that all four extend is the same work with a durable result.

cf-sync-engine already works this way -- one tsconfig.base.json carrying strict, noUncheckedIndexedAccess, isolatedModules, forceConsistentCasingInFileNames, and each package's own config narrowed to just lib, types, and include. That is the target shape, with one difference: cf-sync-engine's base carries noEmit, but shared and db here emit declarations (declaration: true, outDir), so noEmit stays out of the base and web / workers set it themselves.

Done when

• tsconfig.base.json exists at the repo root with the shared flags
• All four packages extend it and declare only what is genuinely package-specific (lib, types, paths, include)
• The union of today's flags is applied everywhere, so workers gains the four checks it is missing
• pnpm typecheck clean

Expect fallout in workers from the newly-applied flags. That fallout is the point of the issue, not a side effect.

Effort: 2-3 hours including the workers cleanup.

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Part of #778 (TypeScript correctness target state and tracker).