Fix migration numbering collision and bump Dockerfile to Go 1.25

The `000016_task_template_id` and `000017_drop_task_template_regions_join`
migrations introduced on gitea collided with the existing unpadded 016/017
migrations (authtoken_created_at, fk_indexes). Renamed them to 021/022 so
they extend the shipped sequence instead of replacing real migrations.
Also removed the padded 000012-000015 files which were duplicate content
of the shipped 012-015 unpadded migrations.

Dockerfile builder image bumped from golang:1.24-alpine to 1.25-alpine to
match go.mod's `go 1.25` directive.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/022_drop_task_template_regions_join.up.sql

@@ -0,0 +1,5 @@
+-- Drop the legacy many-to-many join table task_tasktemplate_regions.
+-- Climate-region affinity now lives in task_tasktemplate.conditions->'climate_region_id'
+-- and is scored by SuggestionService alongside the other home-profile conditions.
+
+DROP TABLE IF EXISTS task_tasktemplate_regions;