fix(server): handle duplicate asset checksums atomically with ON CONFLICT

[Oddly]

Summary

Replaces the check-then-insert race condition in asset creation with PostgreSQL ON CONFLICT DO NOTHING on the UQ_assets_owner_checksum constraint. This eliminates the noisy constraint violation errors that spam server logs when the mobile app uploads duplicate assets.

Fixes #22742

Changes

AssetRepository (asset.repository.ts)

• create() now uses ON CONFLICT (constraint UQ_assets_owner_checksum) DO NOTHING with executeTakeFirst(), returning undefined when the asset already exists instead of throwing
• createStrict() (new) preserves the original throwing behavior via executeTakeFirstOrThrow(), used by createCopy() during asset replacement where a conflict is a real error
• createAll() also uses ON CONFLICT DO NOTHING, so batch inserts silently skip duplicates

AssetMediaService (asset-media.service.ts)

• uploadAsset() handles the undefined return from create() as a duplicate: cleans up uploaded files, returns DUPLICATE status, and skips the usage update — all without logging a constraint violation
• createCopy() switched to createStrict() since backup copies during asset replacement should never conflict
• Private create() method now returns { id, duplicate } for clean control flow

MetadataService (metadata.service.ts)

• Motion photo extraction simplified from try/catch with isAssetChecksumConstraint to a straightforward if (created) / else pattern
• Removed unused isAssetChecksumConstraint import

Tests

• Updated duplicate handling tests from mockRejectedValue(error) to mockResolvedValue()
• Updated replaceAsset tests from mocks.asset.create to mocks.asset.createStrict
• Added createStrict to the asset repository mock

Verification

• tsc --noEmit — clean
• All 2185 tests pass (102 test files)
• pnpm lint — clean (0 warnings, 0 errors)

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 🗄️server. A maintainer will add the required label.

[Oddly]

I've tested the issue before and after this patch, and this patch does seem to fix it. This was definitely LLM assisted, so I took care to write tests an also manually test again.
Could replicate the issue by doing the following:

• Setup Immich
• Fire off 20 concurrent image uploads with the same image
• HTTP responses are fine, but the immich logs will show "multiple constraint violations" with a stack trace.

In the newer version, this is not issue any more.

[Oddly]

I've asked for feedback with a test image in the issue thread.

[jrasm91]

Closing in favor of #26113