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.down.sql

@@ -0,0 +1,12 @@
+-- Recreates the legacy task_tasktemplate_regions join table. Data is not
+-- restored — if a rollback needs the prior associations they have to be
+-- reseeded from the task template conditions JSON.
+
+CREATE TABLE IF NOT EXISTS task_tasktemplate_regions (
+    task_template_id  BIGINT NOT NULL,
+    climate_region_id BIGINT NOT NULL,
+    PRIMARY KEY (task_template_id, climate_region_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_task_tasktemplate_regions_region
+    ON task_tasktemplate_regions (climate_region_id);