[EPIC] Layers from VEDA STAC collections, with open-ended time

[BhattaraiSijan]

Story

Mission admins add VEDA data to MMGIS by hand-assembling tile layer configs
from facts scattered across a STAC catalog — and the hardest fact is one
they cannot know: when the data ends. Most VEDA collections keep
growing; a config written today with a concrete end date is stale tomorrow.
Forecast products are worse — their whole time window moves every day.

This epic makes VEDA collections first-class layer sources:

1. A layer's time range can be expressed as a policy ("ends now",
   granularity-aware) instead of a fixed timestamp, so configs for growing
   data never go stale.
2. An admin can create a tile layer from a VEDA STAC collection: give a
   collection id, see what the collection offers (its render options and
   temporal coverage), make the few real choices, and the layer's fields
   fill from the collection's own metadata — including the open-ended time
   policy when the collection is ongoing.
3. (Follow-up) Forecast collections, whose start and end both move, are
   expressible with the same policy mechanism.

Sub-issues

• Layer time can be open-ended ("now") and granularity-aware #332
• Create a tile layer from a VEDA STAC collection #333
• Follow-up: forecast layers whose whole time window moves #334