fix: let -link-pylovo re-link and report already-linked buildings

[jravani]

Closes #154.

What

• -link-pylovo reports correctly when every building is already linked.
• The new -link-pylovo -relink re-links buildings that already have a building_link row, for example after the PyLovo data changed.

Why

-link-pylovo batches only buildings without a building_link row. That is deliberate, and it is kept: a run after a small import links just the new buildings. But:

• with every building linked, the run logged No LOD2 or LOD3 buildings with footprints found, although the buildings and their footprints exist;
• nothing could re-link after PyLovo changed. The link script deletes and re-inserts its batch's rows, but the Go batching never handed it a linked building, so the only route was truncating building_link by hand.

How

• GetGridBatches takes includeLinked. When it is set, the anti-join on building_link is dropped.
• RunPyLovoRelink runs the link with includeLinked. RunPyLovoLinkBuild is unchanged for its callers: the CLI without -relink, and the on-request server.
• When nothing was batched, the run counts linkable buildings. If there are some, it logs All N buildings with footprints already have a building_link row. Nothing to link; run -link-pylovo -relink to re-link them.
• -relink without -link-pylovo exits with an error.
• The script header, docs/code/pylovo-link/index.md and the README flag table say which run re-links.

Tests

• TestRunPyLovoLinkBuild_ReportsAlreadyLinked: the second run's warning names already-linked buildings. Fails on main with the footprint message; passes here.
• TestRunPyLovoRelink_UpdatesExistingLinks: after the PyLovo row a building matched is replaced, a plain run keeps the old link, and RunPyLovoRelink switches it to the new one. It fails if RunPyLovoRelink keeps the anti-join (checked by wiring it to includeLinked=false).
• TestGetGridBatches_ExcludesAlreadyLinkedBuildings also covers includeLinked: the linked building is back in the batches.
• go vet, the unit suites and the integration suites pass.

Real data (Vienna, Josefstadt; a copy linked over postgres_fdw)

• Plain -link-pylovo on the fully linked copy: logs All 1318 buildings … already have a building_link row; all 1,318 rows keep their created_at.
• -link-pylovo -relink: all 1,318 rows rewritten, identical result: 1,219 matched, 1,198 distinct OSM ids, 15 OSM ids linked to more than one building, 0 changed.

[codecov]

Codecov Report

❌ Patch coverage is 69.76744% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/process/feature_extraction.go	80.55%	4 Missing and 3 partials ⚠️
cmd/c2t/main.go	0.00%	6 Missing ⚠️

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