fix: migrate patch timestamps to timezone-aware UTC instants

[jpenilla]

Replace LocalDateTime with Instant throughout the patch lifecycle, API, statistics, and time utilities. Persist last_updated as a timezone-aware database timestamp and parse backend timestamps as UTC in the frontend.

Add Flyway migrations for H2 and PostgreSQL, including conversion of existing legacy timestamps, and switch Hibernate schema handling to validation.

When upgrading a database created by the previous version, configure spring.flyway.placeholders.legacy_timezone to the timezone used by the old deployment if it was not UTC.

[Copilot]

Pull request overview

This PR migrates patch lifecycle timestamps from LocalDateTime to UTC-based Instant end-to-end (DB schema + backend API + frontend parsing) to make timestamps timezone-safe and consistent across deployments.

Changes:

• Backend: replace LocalDateTime with Instant for Patch.lastUpdated, API DTOs, stats calculations, and time utilities.
• Database: add Flyway migrations for H2/PostgreSQL to convert last_updated to TIMESTAMP WITH TIME ZONE, with a legacy_timezone placeholder for interpreting old local timestamps.
• Frontend: parse patch lastUpdated timestamps as UTC and then display them in the user’s local timezone.

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
web/src/lib/types.ts	Documents lastUpdated as an ISO 8601 UTC datetime string returned by the API.
web/src/lib/components/PatchesTable.svelte	Parses lastUpdated as UTC before converting to local display time.
src/main/resources/db/migration/postgresql/V1__create_patch_table.sql	Introduces baseline patch table definition for Flyway-managed PostgreSQL schemas.
src/main/resources/db/migration/postgresql/V2__convert_last_updated_to_utc.sql	Converts legacy last_updated values to a timezone-aware column using a configurable legacy timezone.
src/main/resources/db/migration/h2/V1__create_patch_table.sql	Introduces baseline patch table definition for Flyway-managed H2 schemas.
src/main/resources/db/migration/h2/V2__convert_last_updated_to_utc.sql	Converts legacy last_updated values for H2 (currently contains a likely invalid H2 ALTER syntax).
src/main/resources/application.yaml	Enables/configures Flyway and switches Hibernate schema handling to validate.
src/main/java/io/papermc/patchroulette/util/TimeUtil.java	Updates interval types from LocalDateTime to Instant.
src/main/java/io/papermc/patchroulette/service/PatchService.java	Updates patch lifecycle updates to use Instant.now().
src/main/java/io/papermc/patchroulette/model/Patch.java	Migrates lastUpdated field and duration update logic to Instant.
src/main/java/io/papermc/patchroulette/controller/ApiController.java	Updates API DTO and stats computations to use Instant.
build.gradle.kts	Adds Flyway dependencies (including PostgreSQL database support).
.gitignore	Stops ignoring the Flyway migration resources directory.

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