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/021_task_template_id.up.sql

@@ -0,0 +1,13 @@
+-- Add a backlink from task_task to task_tasktemplate so that tasks created from
+-- a template (e.g. onboarding suggestions or the template catalog) can be
+-- reported on and filtered. Nullable — user-created custom tasks remain unset.
+
+ALTER TABLE task_task
+    ADD COLUMN IF NOT EXISTS task_template_id BIGINT NULL;
+
+CREATE INDEX IF NOT EXISTS idx_task_task_task_template_id
+    ON task_task (task_template_id);
+
+-- Deferred FK — not enforced at the DB level because task_tasktemplate rows
+-- may be renamed/retired; application code is the source of truth for the
+-- relationship and already tolerates nil.